2 females will fight much quicker then 2 males for some reason.
In most frog species it's better having more males then females if you like more then a couple.

The shape of the body and size of the toepads are just indications,
looking at just that is proven wrong for a lot of people.
Females do tend to be somewhat bigger in most cases then males when fully grown.
Like already said, it could be a fat male.

Altough if i would guess i'd say male left, female right.
Since the right one is build more feminin like and got smaller nuptial pads as i see it in this picture.
Could be perspective ofcourse.

Having a male and female doesn't necessarily mean they tend to spend time together.
Just as with people, they must like eachother to become a good breeding couple.
Best way is to buy a group and check wich males and females are attracted to eachother, you can tell since they will actually spend time together by cuddling up in the same area.

Only guarantee you have is that males call and females lay eggs,
with paying close attention to you;re animals it will be more and more clear what females and males are.
Most will have a certain behaviour you will start to recognize more and more as you get more experience with them.

I would say, try an unisex name on the right one, like René.

Good luck with them!