Hello!
Indeed the SCC-1 will give you a Roland intelligent mode MPU (perfect for MT-32 games) and an onboard synth that is basically a SC-55 (so perfect for GS/GM games).
So you will have to deal with 2 extra audio outputs: the SCC-1 and the CM-64.

One valid solution is what you describe, select which synth goes into the SB Line-IN port - either manually or via some audio-switch perhaps (like this , there are bazillions on ebay etc)

Another better (and more expensive) solution is to get an external stereo mixer, connect all 3 devices to it and adjust volumes on the fly. Once you really start messing with midi in games you'll notice that volumes are all over the place and many games do not offer any kind of volume control. For the CM-64 you can always use it's volume pot to fine tune but for the scc-1 there is no such thing...

... basically you can make it as cheap or as expensive you like with the cost of ease of use: you can just plug/unplug cables or get a switch or a mixer.
Hope this helps!

I recommend to avoid line-in on old Sound Blasters. It can reduce the quality a lot. There are alternatives to using external mixer. I like to use Creative Gigaworks T20 and T40 speakers because you can connect two devices to them at the same time. You only have one common volume control but it has been enough for me.

Friends:

So I got the SCC-1, a simple, powered line mixer, and a bunch of cables.

It's now connected as follows:

1. The SB speaker out is connected to Channel 1 of the mixer.
2. The CM-64 is connected via Midi cable to the Midi Out on the SCC-1.
3. The CM-64 is also connected to Channel 2 of the mixer.
4. The SCC-1 is connected to Channel 3 of the mixer.

Incredibly, it all works. But there is one thing. Whenever I play Midi, whether GS or Mt32, it comes through both Channel 2 and Channel 3. And, of course, listening to Mt32 stuff through the SCC sounds like crap, as does listening to GS through the MT32. So I just mute one or the other on the mixer. But should both be playing simultaneously like that?

I was thinking maybe it was doing some kind of compatibility mode. (Though that still confuses me as to why GS audio would be trying to go through the CM-64.) But I downloaded "SCMODE.EXE" and it says it can't find my midi device. Then I got something called "CHKSCC.EXE" but it won't let me select 330/331 as the address (it says something is on the 331 portion--forgot what it called it.) The card itself is set to defaults--330 and IR2. And the SBPro is set to defaults too--220 and 7.

Maybe I just shouldn't care since it IS working. For example, Day of the Tentacle for example runs general midi fine and sounds good out of its channel and Ultima7 does Mt32 fine and sounds good out of its channel. But as i said in both cases I have to mute the other channel. Is that just how it is? Just my OCD at work.

Thanks!

it is normal that both the SCC-1 and CM-64 play at the same time.
there is only 1xOUT port and it gets "doubled" so the onboard scc-1 synth receives the same data as the midi-out port (that in your case the CM-64 is connected)
you having an external mixer is the best possible scenario and indeed the correct way is to mute with the mixer the synth that is not suitable for usage
so you are all set , nothing else to do than play some good old games 😀

Nice, thanks for your help!

Unless you only have 2 ISA slots, nothing stops you from using the SB Pro 2, the SCC-1 and the Soundscape.

You can configure the Soundscape for a different MIDI port let's say 300 and you'll still be able to access the OTTO2 synth via I/O port 300. Ensoniq Soundscape is mainly supported in games with Miles Audio Library drivers, and probably Sound better than a SB Pro 2 on those games.