Are you taiking for when they are adults.Or Juvniles
For adults Jags they have a nasty temperment and get up to around 15+ AND by itself 90g
And a Jack Dempsey is somewhat or just as aggressive as a jag and get around 12+ itself 75g
for both i think in my honest oppinon A 180+
That's the ballpark I was looking at. From what little I have read it seems that for both good tank mates are agressive cichlids of the same size. I figure if I get them when they're young and put them into the tank at the same time it could be fine and would be a pretty sweet tank in my opinion.
I have own both in the past. They got along until the jags paired up. If the tank is large enouh, add other fish to disperse the aggression. Good Luck.
Female Jags aren't as large as the males more like 12" and a 125 long would be minimum for both a JD and female Jag. I've only ever owned 1 female Jag and currently she's a real livewire, much more aggressive than her larger male counterpart.