config=x activates any MIDI device in my list, properly.
So, when I set config=3, the Yamaha MIDI port is properly accessed when I load the VAPIMPU driver.

config2=x doesn't work at all, even when set to "0" (the Audigy I/O), since I'm unable to load the SAPIMPU driver at I/O 332 to access it. SAPIMPU will only load with it's default setting of IRQ 2/9 330H. Unfortunately, SAPIMPU can't load unless VAPIMPU is FIRST loaded. And, SAPIMPU doesn't allow for specifying IRQ.

If I load SAPIMPU with its defaults (IRQ 2/9, I/O 330), it just uses the same device that's setup under "config=x", so that both SpG MIDI Port 1 and 2 access the same MIDI device.

I've been doing a lot of testing with music files under DOSBox, and there's some noticeable timing issues that make it rather difficult to seriously use DOS-based sequencers under DOSBox. Note-Off controller data seems to be the biggest problem, though.

I believe the problem is caused when the simultaneous MIDI stream reaches a given size. The less data being pushed through the MIDI interface, the better. I'm guessing that there's probably already ways to tweak this in the config file?

When there's too much MIDI data being sent through a sound card's MIDI port (when accessed under DOSBox), some notes hang while other don't get played at all. Using the same port under Windows doesn't present this problem, nor does using the same DOS software on a MS-DOS, Win98, or Win98SE system. Even very old soundcard MIDI ports function fine. But under DOSBox, the load seems to be too much.

It may be extensive use of a given type of MIDI data...such as bender controls (notorious for heavy, redundant placement in MIDI), or various other continuous controllers.

It could be the DOSBox isn't emulating fast enough to pass the MIDI data out. If you're running on a Windows or Linux machine that's less than a couple years old, try these in your dosbox.conf:
- core=dynamic instead of the default core=normal
- cycles=30000 (or at least 10000)

Another thing to try would be running DOSBox's output through MIDI-Yoke/MIDI-Ox. I've had this help with hanging notes before (although it's also made it worse before)

I believe I tried that with the 'cycles', but I'll check. Thanks.

I don't think MIDIOX will work in this situation; when you're sequencing and editing MIDI data, you need the sequencer (SpG, in this case) to be able to play the MIDI data you're working with.