If they've been together for 2 solid years with no eggs produced, here are the possibilities:
--they are both female
--they have bred (when you weren't looking) but --
either the male or female isn't fertile
Your choices are the following:
--separate them for a week and then put them back together to see if you see more breeding behavior
--get another female and (after a quarantine period) introduce her to the male. If you still get nothing, get a new male and introduce him to the female(s).