Back to back closets is ideal. You can have cleats for both closets at the same time. You can just bolt through the drywall, but try to stay close to the studs so you don't crack the drywall if you over-tighten the bolts. Don't ask me how I know this.

I started out with nothing & I've still got most of it left
M60 VP160 QS8 EP350
M22 VP100
Algonquins