neither of these speices live's up to there names in aggresion so i think you could easily keep htose 2 together in a 55, i also find that both males max out at 10"ish- Jd's are bigger up to 12", GT's 8-10".
ohhhhh mine sure did it killed an 7'' oscar at 4'' and then beat the crap out of my other fish so he had to go he did have nice colors i still miss my GT but in a 55 you could try it buts its useually going to work or it will fail try it but be ready to move them if they fight they could do just fine so its worth a try
in the wild GT have no proublem getting over a foot but a GT over 12'' raised in captivity is pretty hard to come by
pretty much the same with jack dempsys except they only get 12'' in the wild hard to find one over 10'' in captivity