I'm agreeing with the case-by-case basis here. But if they're, as has already been said, from different canons within the same franchise (eg different Transformers series, KH vs FF, etc.) I think there's no problems allowing them.

I'm not sure how I feel about doubles from different points in the same canon, but I'd probably say no. :/ That's what dressing rooms are for...

Basically, what a lot of other people have said, by the looks for things. XD