I've got a JD in a 55 gallon and wanted to add another fish. I got a few people at the LFS that said it would be ok. Going to better fish stores looking for that fish to add to my tank, I got the opposite response. I set to the MFK site and joined and received the same answers here- is it possible? Yes. Is it a good idea? Not so much.
Not even whether the 2 fish get along at a later age, at full grown the tank will be overstocked. I then considered going with a JD female for my male JD as females stop growing after they mate. I then was talked out of that by another fish store owner. JD's in the wild are not schooling fish, if you're not planning on mating them (and I'm not), there's no reason to put a female in there. JD's hang out alone in the wild, they don't need company but to mate.
It's very possible your bosses JD and GT will get along because they're growing up together, but to maximize each fishes potential, I think it's important to give more room than 55 gallons to two fish of the size of a GT and JD at full size. There are other possibilities as far as fish go if he's insistent on more than one fish in the tank, but I wouldn't keep them two species he has in such tight quarters (based on full grown size alone).