I set up a site for a theater school and one thing they're selling is day camp for next summer. Parents can sign up their kids for Drama Camp in July, Musical Theater Camp in August, and Both (Drama & Musical). The maximum number of students for Drama is 30 and for Musical is 25. The issue is Both: A) When a parent registers for Both, is there a way for the quantity to drop in each of the other two? B) When a parent registers for either Drama or Musical, is there a way for the quantity of Both to drop?

I saw the "Stock by Attributes" mod, but that doesn't seem to be the answer and it would also be unhelpful in a few ways to sell this as one item with options.

I thought of playing around with the Master Category feature, but that doesn't allow for different-named products to be linked and that would anyway only answer part of the question.

Any ideas?