My experience is that Zoas touching each other on plugs is not problematic. Zoas will however compete for space if on the same rock. The lesser aggressive of the two will loose out.
I have several types of Zoas on small rocks next to each other, one is starting to transfer to the rock next to it, and the other is recoiling a bit. Separated the rocks and all is well again even with the polyps touching each other.
Just my experience though.