help needed w form lookup re: 'set criteria' script

help needed w form lookup re: 'set criteria' script

hi folks, hoping someone can help me out here, I'm in beyond my depth:
Here's my challenge:
I have 2 forms:
  1. the first ('form a') is a list of church services, with various info, by date.
  2. the second (form b) is a list of musicians, with various details, by name.
Form b includes a lookup field (called 'availability) with (multiselect) checkboxes to allow musicians to see the dates from form a and select the ones they are available. no problem so far.

however, form a also needs a field showing which musicians are booked for each date. I have no problem with a lookup (multiselect checklist) to get the list of names from form b, but it would be ideal if it were possible to limit the criteria of names shown to just those who are available on that date.

hoping someone can advise me on how to do this...
Please tell me if you need more information.
Thanks!
Amy