I have my assist being sent with my DPS rotation keys as Step 1. For fights where I need/want to be on 2 targets simultaneously I made a mapped key that toggles that step off using a Mapped Key Step State Action so I can switch auto-assisting off on the fly.

I don't see why you'd need to dedicate your focus frame for this purpose, since you'll most likely still be stuck acquiring the target through assisting your lead and then focusing it. Suppose to each his own though.