Aneesa's Muse :
I'm not trying to start one of the infamous ill debates here, but I have been breeding for years and have had a few Cresteds in my care ...and the female presence has no factor in the fighting, other than increasing the viciousness.
A 23 gallon tank is small for one adult Crested, as well. 
It is always best to err on the side of caution when keeping animals in captivity... they deserve that much.  
I'll edit your post to say female presence may or may not have a factor in fighting, as it seems to depend on the geckos. My cresteds clearly have very different personalities, or aggression triggers, than yours do.
I will agree with the rest of your post, though. 20 gallons is the recommended minimum for one crested, but if you can get them a room sized aquarium that's even better. Bigger is better is true for any animal. And you can keep two healthy in a 20 gallon, if it comes down to it, but I won't vouch for their overall happiness.