Have a solid contract that will detail EVERYTHING:

How the animals are divided the type of split whether it is 50/50, 70/30 etc (split depends on the value of the animals if one is more valuable than the other obviously the person with the most valuable animal will get a bigger split.

Who gets the first pick

What happen if one animal becomes sick, requires a vet and who will pay for it.

What happens if the animal die how will it be replaced.

Etc

Keep in mind that a loan with a pet store can be VERY risky because there is so much turnover animals coming and going and with VERY little quarantine.