Ok. Most amphibians and all Bufonids (true toads) have skin toxins, while at the same time they possess porous skin that absorbs just about everything through it. So if you are keeping two amphibians of different species, you are not only risking cross-infection, but cross-contamination. In other words they would be poisoning each other. You can't quarantine against that.

Now if you have two toads of the same species and one ends up growing significantly larger than the other, then most likely you have a male and female. In most Anuran species the female grows larger than the male.