Long time since I did it, but you need a genuine text only mono, or hercules compatible, or multi card of EGA/CGA/MDA variety that actually turns off it's color BIOS properly when set to mono, don't be thinking a card that does mono CGA output will do it either. These will be ISA and I think exclusively 8 bit ISA. Then that will co-exist with any VGA. Now to get output, you have to reassign console with DOS commands, or have a programming util that supports it, I think borland's stuff did. Also works to have Win 3.x on the VGA and a text console, forgot details. Mono output needs a CRT mono monitor.

However, sometime around mid pentium era, plug and play BIOSes and Win98SE got smart enough to reallocate resources such that two PCI cards or AGP and PCI could co-exist in a system, provided the PCI card let it's resources be managed (i.e. older ones might not work, as a general clue, if it's only got 1MB on it it has more chance of being older) Then also you've got to run compatible screen mode and color depth on each card in 98, so your max is only what your secondary card will manage (presuming you have a slightly better one as main). A caveat is that many drivers didn't support two cards of the same chipset, exceptions might be "pro" cards like Matroxes, so you have to run two different cards... this might have got fixed for cards released after 99 or so. However, when I did this, I had a number of cards to choose from, I believe I ended up using a Voodoo3 2000 and a Trident TGUI9680 together, but it was a couple of decades back. But this all works in windows or linux but I've no idea about DOS for this configuration.

Edit: I'm having a vague suspicion that what happens in the 2 PCI case is that the BIOS on the secondary is disabled maybe, leading to no predefined DOS modes available, and it only knows what the windows driver tells it to do.

VGA and MDA use different memory addresses.
A0000 for VG A and B0000 or B8000 for MDA if memory serves me.
writing to those addresses in the correct fashion, for text mode MDA one byte is the text to display, the next byte is the attribute, following byte text, next attribute etc... (again if memory serves me) text mode 80x25 = 2000 bytes text and 2000 bytes attributes. (attributes being bright, blink, ...)
for the vga, it's the same in text mode, but the attribute is the color that gets displayed.

If you're using windows or desqview, you can just open another window/session and switch to the mda monitor in dos by typing mode mono, I think.

It's been a while... about 20yrs

edit: you could do the same thing with multiple pci&agp cards but there the bios starts to become annoying. yes you could remove the bios, but then you will have to set each card up (usualy done by an appropriate driver) to the correct video mode, text or graphics) and write your data directly to the memory address of each card. going from the assumption that each card should have gotten different addresses and the addresses can be found by some pci magic I know not enough about. 😀

More info for win98..
http://www.helpwithwindows.com/windows98/multi-monitor.html
and this is the now dead KB link in there.
https://www.betaarchive.com/wiki/index.php/Mi … _Archive/179602