Dear Carlz,
I would agree with AxolotlChris. The larger one appears to be male. The smaller one might be female or a late bloomer - it's best to wait until they're a year and a half before trying to guess.
Don't worry, they're not skinny at all
Tummies should be around as wide as their heads. The little rib markings are also normal - even the plumpest have them.
Axolotls can be very sociable animals and, once used to each other, do like touching or 'stacking' regardless of sex. I think it makes them feel more secure.
Courtship is quite subtle and can go completely unnoticed - the first sign I ever got was eggs in the tank! Axolotls don't mate like some other animals - the female releases a hormone that makes the male drop spermatophores, which look like tiny white hills, inside the tank. The female will pick one of these up and fertilise the eggs inside her body. She will then lay them around 12-24 hours later.
Sometimes the male might nudge the female a little or there may be some 'nosing' around the bottom areas, but they don't generally show any other signs of courtship. Pairs can form between axolotls of either sex and they do form attachments to those tankmates. That's why it's a good idea to separate males and females early if you don't want to deal with spawnings.
