I still think that you're better off running split Audio/Midi tracks, rather than Simple Instrument Tracks, simply to avoid these sort of problems.

If it works for you using split then there's another reason not to use SIT's (apart from all the other well documented reasons)

Me...I've never used a Simple  Instrument track - I don't know what they're really about. I wouldn't feel comfortable commenting on them. I'm guessing they're a combo of a midi track and the main audio out the assigned synth??

I suppose because I'm somewhat comfortable with the notion of manually patching a midi track over to a midi channel of one of the synths in the rack, and then manually assigning the synths audio out channels over to audio tracks. That's the old-school flexibility that attracted me to Sonar, after having to use a synth-per-track in Acid in order to use effects on a track basis rather than the entire synth. Apparently, Acid doesn't do multiple audio-channel synths, AFAIK.