At some point, I was thinking that we would need a sequencer per sm:
spsait_sm1, spsait_sm2 ...
My point was mainly to simplify the commands, because in that case I didn't need to parse spectrograph module ID

But Craig suggested that in fact, we need only one sequencer and a place to store the current hardware configuration (ics_config ultimately) .

that would give :
sps=sm1,sm2
sm1=blue,red
sm2=blue

by default, If I do an exposure, spsait will know which spectrograph and which arm to command.

I think that's fairly simple and flexible.