Corals of the Euphylia genus are most dangerous to corals of any other genus. To each other, most of the time, they're pretty tolerant. I've seen torch, frogspawn, and hammer all bunched together with tentacles intertwining and no problem. They seem to be able to recognize their relatives. I've had a torch and frogspawn touch, with no problems (though some nearby mushrooms got brushed and were NOT thrilled) and right now, I have a little branching hammer frag that gets brushed by the frogspawn every few minutes. And the branch that gets brushed is the one that opens the fullest, so it seems not to mind getting felt up by it's cousin. LOL
Anyway, there is always the possibility of Euphylia corals stinging each other, but it's far more likely that they'll attack any non related corals that are nearby.
Someone stop me! Please! But... not just yet...
55 Gal Tropical FW
55 Gal Mixed Reef