What you're looking for is a stereo audio switch or mixer. You can set up a mixer to permanently combine both signals and just leave it alone while with a switch you need to press a button (or turn a knob or what have you) to switch between sources.
Both are avaliable in a wide range of prices and quality. I'm certain you'll find one that fits your needs with those key words.😃👍

JustRob wrote on 2021-12-20, 20:05:

Maybe not necessarily "old hardware", but... I have Windows 98 SE PC with an ISA sound card I use to play dos games, and a PCI sound card I use to do Windows stuff. It's a bit tiresome plugging over my speakers each time I wanna do one or the other.

I've ordered two different 2x 3.5mm male to 1x 3.5mm female from Amazon so far, but with both of them, one of the male plugs is for microphone, and it does not work if I plug it into line input.

Tried searching around a bit more, but the only ones I can find are of this same type, and none with explicitly two male line input plugs. Does such a thing exist, or am I just gonna have to keep switching my speakers from one sound card to the other?

There are plugs like that, I just ordered one myself for the same purpose. Took a while to find one since like you said, they are almost all those headphone + mic things. I got this one from Amazon.de: https://www.amazon.de/-/en/gp/product/B07CZYRPG9/

I could use a good switcher too though, but I'm not loving all those chinese craps they all seem to be. And then there's the Schiit SYS which looks nice but is ridicilously expensive.

On my setup I plugged one computer to the line in on another

Sombrero wrote on 2021-12-20, 20:30:

JustRob wrote on 2021-12-20, 20:05:

Maybe not necessarily "old hardware", but... I have Windows 98 SE PC with an ISA sound card I use to play dos games, and a PCI sound card I use to do Windows stuff. It's a bit tiresome plugging over my speakers each time I wanna do one or the other.

I've ordered two different 2x 3.5mm male to 1x 3.5mm female from Amazon so far, but with both of them, one of the male plugs is for microphone, and it does not work if I plug it into line input.

Tried searching around a bit more, but the only ones I can find are of this same type, and none with explicitly two male line input plugs. Does such a thing exist, or am I just gonna have to keep switching my speakers from one sound card to the other?

There are plugs like that, I just ordered one myself for the same purpose. Took a while to find one since like you said, they are almost all those headphone + mic things. I got this one from Amazon.de: https://www.amazon.de/-/en/gp/product/B07CZYRPG9/

I could use a good switcher too though, but I'm not loving all those chinese craps they all seem to be. And then there's the Schiit SYS which looks nice but is ridicilously expensive.

Ordered this. It works, but the sound on my ISA card is incredibly low when both plugs are plugged in.

Guess we’re gonna have to do it the old fashioned way.

This one works for me https://aliexpress.ru/item/4000529145663.html

The old way of doing it would be to use a very short 1/8" stereo phono cable into the Line-In of the other sound card and make sure that the Mixer is preset at the start for the card the previous is going into to balance the levels to be even between the devices. IE, say I had a WSS card and an old Adlib card, the Adlib would hit the WSS card through LIne-In, and then at startup the WSS mixer utility would set the levels so the Adlib and WSS are at the right balance I wanted.

JustRob wrote on 2021-12-22, 16:53:

Sombrero wrote on 2021-12-20, 20:30:

JustRob wrote on 2021-12-20, 20:05:

Maybe not necessarily "old hardware", but... I have Windows 98 SE PC with an ISA sound card I use to play dos games, and a PCI sound card I use to do Windows stuff. It's a bit tiresome plugging over my speakers each time I wanna do one or the other.

I've ordered two different 2x 3.5mm male to 1x 3.5mm female from Amazon so far, but with both of them, one of the male plugs is for microphone, and it does not work if I plug it into line input.

Tried searching around a bit more, but the only ones I can find are of this same type, and none with explicitly two male line input plugs. Does such a thing exist, or am I just gonna have to keep switching my speakers from one sound card to the other?

There are plugs like that, I just ordered one myself for the same purpose. Took a while to find one since like you said, they are almost all those headphone + mic things. I got this one from Amazon.de: https://www.amazon.de/-/en/gp/product/B07CZYRPG9/

I could use a good switcher too though, but I'm not loving all those chinese craps they all seem to be. And then there's the Schiit SYS which looks nice but is ridicilously expensive.

Ordered this. It works, but the sound on my ISA card is incredibly low when both plugs are plugged in.

Guess we’re gonna have to do it the old fashioned way.

While that cable works for you, it is generally considered a 'bad idea'tm since you're connecting two outputs directly to each other and the speakers.
Now what happens when you unplug the speakers.. two outputs connected to each other.

I guess so, it's trying to put out one signal to two different cards, which reduces the volume. Although the PCI card is only used in Windows, and the ISA card only in pure dos. I don't know when it initializes the PCI card, and if it's possible to disable it only in pure dos. Doesn't seem like it.

So uh, just if anyone's curious or has had the same problem, I found the solution. Had a cable lying around with a 3.5mm plug on each end. Plugged one end into the line out of the PCI sound card, and the other end into the line in of the ISA sound card. Works like a charm.

JustRob wrote on 2021-12-25, 12:41:

So uh, just if anyone's curious or has had the same problem, I found the solution. Had a cable lying around with a 3.5mm plug on each end. Plugged one end into the line out of the PCI sound card, and the other end into the line in of the ISA sound card. Works like a charm.

That’s what I said to do 🤣

Sphere478 wrote on 2021-12-25, 12:50:

JustRob wrote on 2021-12-25, 12:41:

So uh, just if anyone's curious or has had the same problem, I found the solution. Had a cable lying around with a 3.5mm plug on each end. Plugged one end into the line out of the PCI sound card, and the other end into the line in of the ISA sound card. Works like a charm.

That’s what I said to do 🤣

Oh… I completely missed that. Sorry 😑

JustRob wrote on 2021-12-20, 20:05:

Maybe not necessarily "old hardware", but... I have Windows 98 SE PC with an ISA sound card I use to play dos games, and a PCI sound card I use to do Windows stuff. It's a bit tiresome plugging over my speakers each time I wanna do one or the other.

I've ordered two different 2x 3.5mm male to 1x 3.5mm female from Amazon so far, but with both of them, one of the male plugs is for microphone, and it does not work if I plug it into line input.

Tried searching around a bit more, but the only ones I can find are of this same type, and none with explicitly two male line input plugs. Does such a thing exist, or am I just gonna have to keep switching my speakers from one sound card to the other?

The cables you got probably has 4 poles on the female side. That's for connecting smartphone headsets with built-in mics (using a single 3.5mm 4-pole connector) to sound cards and obviously won't work.

Many years ago I made similar attempts by combining several audio cables to form 2x3.5mm male and 1x3.5mm female (all 3 poles) but it did not work well. Only one output worked correctly, while the other was severely distorted.

In the end I got myself a mixer to manage the many sound cards that I would connect to a single set of speakers. All I needed to do was connecting the stuffs to the mixer, configuring the input and output levels, and it'll work like a charm without any configurations on the PC side.

There are many kinds of mixers. Some are very simple and easy to set up, while others can be very big and complex and come with many adjustable options.

I do recommend using a simple stereo mixer for connecting sound cards (input) to a single set of speakers (output). RCA jacks are preferred, as from my experience, 3.5mm male-to-male cables don't always work correctly.

Should note that if the mixer you use doesn't have built-in amplifier you'll need to connect the mixer output to an external amplifier before sending to the speakers or it'll be very quiet.