African Side Necked Turtles

I have 2 african side necked turtles which are small. we are not sure how old as the guy in the pet store wasnt sure. we think 1 is a boy and 1 is a girl. 1 of them is more colourful underneath and seems more dominant, the other is dark underneath and has a smaller tail.

i am a first time turtle owner so i am learning more things about them each day.
both of my turtles occasionally bring their head out of the water for a few seconds and then go back under. i was just wondering if this was normal?