Can you keep 2 male turtles together?

I have two male turtles. They have been raised together since they were babies. They are now 9 years old. One is a Red Ear Slider and the other is a Western Painted Turtle. They were my brothers and he has given them to me, I got a 275 gal pond for them and it is kept inmy house. The problem is that the little one which is the painted turtle keeps biting on the big ones neck. I have seperated them, I put a divider plastic fence in between them that they could see each other but couldn't cross. They get really depressed and won't leave the fence. All they want is each other. I want the little one to quit biting on the other. Any ideas?