Every now and then, I find my turtles going to the highest point in their water container with one climbing the other and fully stretching out their heads.They stay this way for quite a while. It is cute but looks weird as I have no expertise.
They stay still all the time and are misaligned for copulation. So it is not that.
What are they doing? Scouting? Sea nostalgia? Daydreaming about being celebrities and Posing for camera? Ventilating?
They are basking in the light at the top of the tank. Basking is a normal and important turtle behavior that is involved in temperature regulation, Vitamin D production, and just drying off for reasons of shell health maintenance.
Turtles tend to go to the warmest and brightest place in the enclosure to bask, and the timing and amount of basking depends on temperature, light level, and season. For a pet turtle that means generally -as close to the heat and light source as possible-. So your 'top' turtle here is taking advantage of his buddy as a stepstool to get just a little closer to those sweet, sweet UV rays!
This article describes basking in more detail and provides a photo of turtles in the wild basking in a stack: http://www.thesuperfins.com/why-do-turtles-bask-in-the-sun/
