The acro will probably sting the part of the monti that touches it. It shouldn’t kill the whole monti, just the edge touching. What you can do is frag the outer edge of the monti by snapping or cutting a piece off when they are about 1/4” apart. Or you can leave them, and let them establish their territory. After a while, it’s like tending to a garden by deciding where to prune or not prune. Looking good!
Here’s one of my tanks for a reference of what I mean.