Not sure if this will help with the separation problem, but one tip I ran across is: lay down the last sheet of phyllo on top of a buttered sheet, but don't butter it; instead, give it a light spritz of water. Add nuts. Spritz the nuts with water. Lay down another phyllo sheet, butter, phyllo, butter, phyllo, etc.
The idea is that the nuts are encased in a pocket of phyllo whose inside is unbuttered, but faintly damp. This is supposed to help the sheets adhere to the nuts a bit. As such, it also keeps the upper sheet from slipping around on the nut mixture while you try to butter it.
Also recommended: spritz the very top sheet just before putting in the oven, which will keep the trimmed edges from curling.
Cutting down the thickness of the nut layer may also help. It doesn't stick to itself well. Maybe cut it in half and use fewer layers of dough in between.