If you can have the 10 it would be better to divide that one....unless Salem objects to the size. And leave the 2.5 up and running should he not do well with such a close neighbor. I have divided tanks so I am saying this from experience. BTW, you do not have to divide the 10 exactly in half. You could divide it so one end is 2.5 or thereabouts and the middle is 7.5. Then if you find another you like you can add a third divider.
RTST The Enabler