Wanting to add that I agree with others above- the easiest/simplest option would be to limit one pair per user per opening. It would also be the easiest to enforce (search a username on the nursery page and they should only appear once).
However it also creates issues of its own, which others have addressed- that of prioritization of pairs, as fables have 2 slots.
An example to illustrate a potential challenge of this: if I have a pair with user A and user B, and prioritize my pair with A, then B must prioritize another pair with their fable- say with user C. What if user C already posted a different pair/told user B that they are prioritizing their other pair? Now user B has to find another pair to post or miss out on the opening. Ik this isn’t the most likely scenario but still something to consider.
A potential middle ground could be to allow a user to be named twice- the maximum number of fable breeding slots- so that they can potentially have both of their pairs posted and no more/ensure that even if they aren’t prioritized they can have a pair posted in an opening. This would be more difficult to enforce, but just a thought!
Whichever way you decide to go, thank you so much bezel for hosting the twin breeding, for engaging the community for feedback, and for running fables!!! I’m so excited to see this species grow, and love all the fun stuff that’s been done so far <33