Could it be that what you have is 2 female blue tongues? If you never see a confrontation between them, that is quite unusual. When breeding time comes around, most females will chase off the male when she has conceived - or if she isn't receptive to begin with!

How long have you had them?
How old are they?
How do you know they are male & female?
Do you keep the temperatures and lighting schedules the same year round?

~Morgana