I understand that things like this can arise, and in some cases there are instances where one character from a spin off would be wildly unlikely to know of a character from the original or another spin off. But, I think that the chances of this are more so on the likely side. This is not just to keep people from character hogging, but also to keep self-cesting to a minimum which is usually always impossible to keep it 100% gone or for it to be reasonable for it to not happen, or there could be cases made to where x character could interact with y because of some reason, but both x and y are played by the same person.

If I let one person do it with an argument that was valid and had to turn another person down, then things could get messy pretty fast, not to mention with canons that are still active this could be a huge problem if an interaction in spin offs do arise (weirder things have happened). So I am going to have to say no, that one character per canon is going to extend to the canons universe as well.