I would keep them in the smaller tank until the female berried up. I'd then move the male and wait for the female to shed the young and move her. If you're lucky you should end up with a number of small CPOs in addition to the two you have now.
All crayfish tend to take up residence near cover and will walk around at night. In a 75, your CPOs would have to accidently bump into eachother at just the right time for reproduction. The chances of you living would be slim.