Most likely bullying a little to young for anything else, as for getting hurt as long as it does not get violent they won't get hurt, but the other probably won' like it.
What do you mean by not do well? He's always been doing laps and eating well. He'll come to the glass and wait for more food. This is the first I have seen the largest one act this way. I had no idea that Tortoises were very aggressive towards each other.
Is 2 years to young to id sex? Maybe the little one is a male, in that case I can separate them as they are all siblings.
In the wild the less dominant one (on the bottom) would quickly leave the area to get away from the one who is picking on him. In the confines of an enclosure, he/she can't get away. This causes high level long term stress and eventually it will affect his/her health. Loss of appetite, weakened immune system, reduced ability to fight off disease or parasites... Eventually, the stress can kill them one way or another. Think back to when you were in grade school. Remember the school bully. How would you like to be locked in a room with that guy? Permanently? Doesn't sound fun, does it? Torts are solitary creatures most of the time. Some species tolerate each other better than others, but they are all individuals. You got away with keeping a pair together for a while, but they are now telling you loud and clear that its time to separate them.
Have you tried to sex these two? Some times males will get an early start and hump anything that looks like another tortoise. Judging by the position of the tortoise in the picture I don't think it's bullying in the sense that he's trying to pick on the other tortoise. I think he's just eager to try out his new toy....oh boy can't believe I just said that. Anyway, it can still be stressful on the other tortoise, so separating them is a good idea.
I tried to video the events and the bigger one's vocals aren't grunts yet. Just squeaks of delight. Too hard to keep the camera still when you're so surprised your laughing.
There are differences in tail lengths. Is it true that females retain the button sized tail of their youth and walk with it outward while the males walk with their tail tucked in?
If keeping one tort is best, I'm sure I can find a good home for the aggressor.