Im with Cerianthus and his wild guess as well... I think the first photo is a female and the second photo, a male.
I believe if they were both males they would have been fighting by now to the point of having to separate them. But maybe that was just the case of my two mature males. Possibly two females can get along better together which may be the reasoning behind the guess's that they are both females.
IMO the second photo showed more of the hump associated with the males than the first photo that appeared to have a slanted forhead.
More photos sure would help!

In any event... if they have been getting along all this time together Id say count your blessings!
