First off, your PBT is just establishing himself as the dominant fish in your 40 hex world.

Knowing how PBTs are famous for developing ich under stress, I'd say it's best to find a way to separate the two ASAP or you run the risk of your quarantine doing more harm than good. Since the PBT is already showing he's boss, your lawnmower would probably fair best if he gets the head start of at least a few days, but that's a judgement call that has LOTS of factors...

Did you get the two at the same time? How long until you were planning to release one into the main tank? How is their health looking so far? How long have they been in QT?

Best of luck with these two. Have heard of some isolated incidents of lawnmower blennies and tangs not getting along so great, just ask hobogato about his blue hippo's love bites! He'd probably recommend you just get rid of the lawnmower and keep the tang...