As they are intact, one (or both) of them may be going through the motions of a phantom pregnancy. Little article
here. In itself, it's not usually something to worry about but they can get very possessive of absolutely anything and two girls can spark each other off for no apparent reason.
I have a 9 year old Westie who cannot be spayed for medical reasons. She has a phantom each cycle and carries her favourite toys to their "whelping box" every night.

Wimmin eh?