Dynamic Availability

Logic for availabilities to communicate/ interact with one another. Once X is booked Y becomes unavailable Once Y is booked X becomes unavailable. We are looking to apply this logic to two separate products using two separate events and availabilities The current constraint we have is differing attendee limits on the two events. I would like Y to become unavailable when 1 attendee books X, rather than once the 20 attendee limit is reached.