Any exposure has already happened and if Max is going to get sick, he will regardless of whether you separate them now. Since they are much happier together, they will be healthier together as well.
I'm sorry you had such an expensive introduction to owning a rat - but I sure am glad you found a good vet right off the bat! They are few and far between and almost always worth their weight in gold. Yes, unfortunately, rats are not always the least expensive of pets. The least expensive part seems to be their purchase price (if any!). I'm glad you've found your way here - you'll find lots of experienced owners and breeders, as well as an occasional vet tech or vet student. And quite a few new rat owners as well!
Scout
Minneapolis MN USA
The purpose of life is to enjoy it. The 14th Dalai Lama