Ian,
It's a bit of a juggling act sometimes, but the more important task usually takes the lead ... the gradual crescendo of the strings may mask the absence of sustain till you have time to switch back. Another fabulous layering effect I've used is a velocity triggered tympani to a piano/string patch ... only the very loudest notes make the drum sound heard, and it is SO cool when it comes in on cue. Very dynamic for show tunes and power ballads.
Gotta run ..... the big band goes on in just about an hour!
(it's SO nice just walking in, hanging my Bari on my neck and walking up to the mic stand! I was the leader or soloist for so long, I forgot how nice it is to NOT have to set up the PA!)