If you put both tanks side by side, put a divider in there so that they can't see each other through the tank walls. Not sure if you are using glass tanks or not but I put two side by side on a bookcase a few years back and woke up one night to some noise in the living room. They were trying to fight each other through the glass! I was afraid for a while that one or the other may cause itself an injury before I could put a book between them. Later that day I went into the kids' supplies and taped construction paper around the sides of the tank.

Bloodsong