You can let them touch and see what happens. The loser will develop STN at the point of contact plus a few millimeters, but the rest of the colony will be OK. The area of STN will develop algae, and kinda looks ugly/dirty, so I usually frag the ends off both coral. One unusual thing that happened at one time was that my Acropora yongeii did battle with the adjacent Acropora tenui, the tenui lost and the A. yogeii started to grow on the tenui bare skeleton!
I subsequently transferred the large tenui colony to another rock to give it room. I must have dropped a piece of it, because a new tenui colony started to grow between the A. yongeii and an adjacent A. millepora. Just for fun, I trimmed the larger colonies to see if the newborn tenui wil grow, and it has!! Here's a photo of the baby tenui growing between two giant Acropora colonies: