i dont know why i never tried but can you keep 2 corals of the same species side by side? i am thinking about soft coral and lps, but i guess sps too. i think you can but i want to be sure before i try.
I've tried it with digitata. I merged a purple digi with a green one for a short period, but the purple eventually became dominant. I have seen one case where some fused a pink prostrata with a yellow one. The colony looked amazing.
Generally that should be OK to keep 2 corals of the same species close together. Chemical warfare is usually waged between different species of corals. I think that's what you were getting at.
Although, I have had different species of SPS grow together, too.
I keep my LPS far enough away from desirable things so that the sweeper tentacles that the LPS won't reach. Some of my LPS put out sweeper tentacles 8" long! 8O
IME green is the most dominant of the different M. digitata color morphs. They can sting each other. I've seen colonies grown together like Kratos described too.
Short answer: Who knows? Stick em next to each other and see what happens!
In the circle on the left is a Brown (Purple-Tip) Monti encroaching on an Orange. The Orange is successfully fighting off the Brown and the Brown is dying where it is close to the Orange.
In the other circle the Brown is encroaching on the Green. There is no fighting at all. The Brown is growing over the Green and they seem to be merging. After close inspection they did not actually merge, they are just very very close together but there is a visible line of seperation between them.
I also keep 2 frogspawns, a torch, and a hammer all together. Interestingly enough they RELLY expand a lot of sweeper tenticles onto each other, but they dont harm one another. As a matter of fact they seem happier, and expand a lot more when they are all together.