QTing can be just for the remainder of your collection but it can also be between QTed animals. Again like many husbandry choices it depends on how much risk you want to take.
If you have a quarantine setup and put 3 animals from 3 different breeders in the same quarantine setup you run the risk of one of the animals having an issue and "infecting" the other 2 animals. So QT can be used not just to keep your current collection healthy but also from other animals in QT. Again this is just a choice just as QTing is a choice.
So while QTing new animals all together protects your current collection...it doesn't protect the QT animals from each other.
That being said even with a bill of health from the vet I believe you said he hadn't run a fecal exam on them yet (e.g. poop exam). So what this means is you really don't know for sure that by breeding they aren't spreading something between one another because they haven't gotten a full "clean" bill of health back from the vet.
Now if you got both of these animals from the same breeder then there is a lower risk of one infecting the other....simply because chances are good if one has something already the other does too. But generically speaking it's a good idea to make sure animals are 100% healthy before breeding them......again though everyone is entitled to take as much or as little risk with their collection and breeding as they want.
I hope that makes sense and is helpful for you.