No, they can touch and all will be fine. I have a digi and a cap growing together as one even. Starting to get green stripes in the orange cap from the digi. You will be fine, just watch at first to be on the safe side.
They should be fine if they are the same species, color does not denote a different species. Much like LPS if they are the same species like Euphyllias they can all be put together and have minimal to no interaction between the 2 seperate colonies. I assume SPS to be that way as well. Unless the specific colonies do defend themselves from other colonies wither it be same species or not, but I would assume they wouldn't recognize the separation. I could be wrong as well since I have only a few SPS......
one problem is that you can't be certain that they are the same sp., as prostrata and mille's can be near impossible to differentiate, and color variance can hold with it chemical warfare variance, i.e. pink pocillipora tends to dominate other colors when in close contact. i also have a bright pink digi that digests any other monti it comes into contact with, and also stings some acros
Twon hit it on the head. I've had several of what I thought were mille's together, but some caused some serious damage to others. Some were likely prostrata and some were likely millepora.
My basic rule of thumb when it comes to sps is that unless I've got two frags of the exact same colony I will not let them touch. The colored sticks seem to be a bit pickier than most corals.