Stacking 'Main lug' load centers for garage
Be aware that both hots and neutral must be in the same pipe.   

Don't butt them against each other/use bare holes. Set them far enough apart that the covers won’t interfere. Use a metal conduit nipple between the panels right wall to left wall. Better yet, use 2-3 sensibly sized ones, so you have an alternate path for crossover wires. If you have 2 metal nipples between them I am very comfortable wiring the ground only to the first one and letting the metal carry the ground to the other.    Can't do that with PVC! 

If you use PVC outside, swiff-sand it with a green Scotchbrite pad, prime it with alkyd primer (Kilz will do) and paint it. That protects it from UV.  

If in the same building

You don't need a shutoff switch so don't waste 2 breaker spaces.  

You can't put 2 wires on a lug.  But you can get 3-wire Polaris connectors and split.  That's what you do with the hots.  For the neutral and ground, if your buses have room for 2 large wires, just daisy chain.  

If in different buildings

Then you need a shutoff switch.  You can backfeed a double breaker to make a shutoff switch, if there is a factory authorized tie-down kit for it to hold it down.   

That takes care of the first panel.  For the second panel, just use the feed-thru lugs on the first one to go to the input lugs on the other one,  just as you have planned.  Come to think of it, they are the same lugs, we just changed one's function by back feeding the panel.