Can anyone sex my red claw crabs?

[zorufoxthing]

I apologize if this is super obvious, but I really can't tell what the crabs' underside should look like. I got these red claw crabs for my birthday and my friend didn't ask for their genders or anything, and I don't want to place two males together since I know they get really territorial, and I'm still in the process of setting up their larger tank (this was a big surprise and I wasn't prepared for it, haha).
Anyways, I've attached photos of both their undersides. If anyone is an expert or something, please help me out. Any input is appreciated honestly. I just want these crabs to be happy.
Also, I understand not to handle them this way, I was very gentle and only did so to snap a quick picture. They're fine now and just chilling out. :]

 

[Autumnsky]

This looks helpful. Maybe you can exchange one.

oops missed the link....
http://krabben.heimat.eu/Englishkrabben/Dateien/geschlecht.html

 

[zorufoxthing]

Edit: the link works! My bad.
I guess I could try and exchange them. Oddly enough, if they are both males, they seem to be getting along well, despite being in a temporary small tank. I wonder why.

 

[Autumnsky]

Oh, good.

When crabs (and fish) are younger they haven't reached maturity and aren't always as aggressive. When maturity hits then they can get the instinct to stake claim /preserve their territory and protect it as well. Often hear, my fish get along fine...and it is because the person has had them a short time, not fully matured.

They need a certain amount of room /territory.

As pet keepers we have to learn what that means. One animal might not need much room and another might need 2x as much as generally accepted as appropriate. That is my thought.

Many inverts like crabs and crays will eat their counterparts if they have the chance -after molting is an especially sensitive time.