Agreed. As long as they aren't acting stressed it won't hurt to keep them together as long as you give them periodic breaks.
Some just aren't as aggressive breeders as others. A lot of the action will take place when they are alone, usually at night when they feel safest. Most locks you probably won't see. What kind of substrate are you keeping them on? If its newspaper look out for spots of dried on sperm. That is a good indicator that they are simply shy breeders. Another method you can try is introducing another male or a fresh shed from another male. That seems to drive them wild.