You should think about separating them. My motto is better safe than sorry. Like untsmurf said, fluttering is frequently an early sign of aggression. They may not actually start bitting each other for awhile, but chances are they will eventually and it would be best to separate them before any damage is done.
I noticed a thread on here the other day about two turtles that had co-habited for 6 years, then suddenly some really serious biting got started and they had to be separated. I guess one of them finally just had it with the other.
