I have never seen a configuration page to switch between different instances of AMPMIX devices. You can switch the default WAVEAUDIO device but strange enough that does not seem to have any effect.
I spent most of today experimenting with UniAud, UsbAudio, and MMPM2.INI, and discovered some things that surprised me. Since I know nothing about the drivers' internals, I'm hoping you can confirm or refute my findings.
It appears that the WaveAudio and AmpMix components of a driver operate independently of one another or at least with minimal dependencies. Without changing the WaveAudio component, I found that I could switch the audio output from my built-in soundcard to my USB audio dongle simply by changing the default AmpMix, then rebooting. Conversely, I could switch between WaveAudio components and always have the output appear at the default AmpMix device.
To confirm that I could actually mix-and-match audio driver components, I wiped out all MMPM2.INI references to one driver's WaveAudio and the other driver's AmpMix so MMOS2 wouldn't (shouldn't?) be able to use them. After a reboot the results were identical to using [defaultnames] to set the components it should use. (Note: I did this multiple times in multiple ways and it always worked as expected.) However, while It was easy enough to confirm which AmpMix was in use, I didn't have a way to prove which WaveAudio was being used (e.g. playing a .WAV that UniAud can't handle but UsbAudio can).
If we can really mix-and-match driver components, then the enhancements you and Wim have made to UsbAudio's WaveAudio component can be enjoyed by everyone, not just those who have a USB audio adapter. Someone with a working UniAud setup could install UsbAudio, then make its WaveAudio component the default. Because of the deficiencies in 'STPM.EXE' (the "Multimedia Setup object"), UsbAudio's enhanced sound would still be routed through his soundcard by UniAud's AmpMix. For many users, it would be the best of both worlds.
BTW... to clear up some possible confusion: it appears that when two drivers are listed for a component, the first is always the default. You only need an entry under '[defaultnames]' if you want to use the second driver for that component. Alternately, you can just change the order of the driver names to change the default.