If they are still very young, say less then a year old, they may just be playing. It's all an instinctive game for youngsters, they probably don't know why they are doing it, it just feels good. But if they are older, and both the same gender, it could be a territorial issue. I'm sorta going thru to same thing with 2 girls and a male and lately it seems to be escalating to the point where I had to move the male to a new habitat for fear he may get hurt by one of the females.
Those who are thinking of getting 2 (or more) turts in one tank need to take into consideration the time may come when the "playing" could turn into something else altogether, and one turt (or both) could get seriously hurt to the point that they may need serious medical attention ($).
Here's a thread going about it on now:
http://www.redearslider.com/forum/viewtopic.php?t=10669