Right now you can only determine one second pet that appears if you call a pet - the pet in the first slot in your stable. However there are 3 pet types, so it would be nice if you could set things up so the second cosmetic pet that appears when you call a pet, is linked to the pet you call. That way you don't have to live with mismatched pets if you change pet spec.
The easiest way I can think of to do this with the current UI is to use the top left corner slot in the pages of the stable. Pet slot one also calls the top left pet in the first page, pet slot 2 calls the top left pet in the second page etc.
One of the best things about BM is the two pets, and matching them to taste. This would give nice flexibility with that. Right now if you change pet type, to have a matching second pet you have to visit a stable and change pets around manually.
I preferred the method in Legion of having a globe earned through a quest line to mirror your second pet. I carried it with me an when ever I changed pets, I used the orb and they matched. When I tamed a new pet, I matched him with the second pet immediately. Now I have to tame two of each pet I tame and my stable is over flowing (never a problem before). I run around with mismatched pets until I find a stable master. Real drag!