Long as it's not too fakely written. A lot of erotic stories strain credulity too much to really get into. As long as you make it believable I'm sure it'll be awesome.

Personally I'd lean toward just one at a time, actually, now that I think about it. Like older sister or younger sister, either way, one on one makes it easy to develop the relationship between them and keep everyone straight during it.

Also... why not twins? If we're talking about animals anyway, they could be part of the same litter, right?