No. Each kit has a unique code that can only be registered once and is assigned to a single pet. If you mix saliva from two animals, the laboratory won't be able to analyse the sample, and a repeat test won't be free of charge.
If you have several pets, buy a separate kit for each one: you'll get a 5% discount from the second unit of the same product onwards. For now, each pet also needs its own account registered with a different email address. Choose the Starter test for dogs or cats, or the Advanced test for dogs or cats.