I am slightly confused as to why you are focusing the NPC, or do you in fact mean targetting it?

If not, getting a toon to focus something should be fairly straight forward. Personally I would just do it in 2 steps. I assume that your assist now works? Then the only necessary step would be to bind a key (one that is also set to be broadcasted to your other window) to "Focus Target" in WoW for the slave toon (or both). I believe it is found under Targetting in the key binds menu. Then target whatever on your lead toon, press your Assist and follow that up with your Focus Target bind.