I have done this already before and have shared my other ideas including this one a while back on previous threads.
Anyways, here is the pic of mine posted before.
This is exactly the same set-up that I use. Really simple procedure:
Remove legs from 22"
Remove dampers from 22"
Remove lid from 18"
setup fire and wood chunks on the fire grate in the bottom of the 18". This takes some experimentation but I normally start with about 14 -19(?) coals in a pile on one side and then make a string of unlit briquettes around the edge of the grate with one end of the string in contact with the lit coals. They will gradually light and eventually, over a couple hours the whole string will have ben lit and burned. You can add wood chunks intermittently along the string of coals or add periodically as desired.
Place the 22" on top of the 18" with the fire grate in place if you want to use a drip tray. Put your meat on the 22" grill, close lid.
Open top vents about 1/2 way
Open bottom dampener on the 18" about half way.
Adjusts vent/dampener as needed for desired temperature.
To check coals or add more coals or wood lift the 22" off and set aside. I use welding gloves to grab the outside of my 22" because it will be really hot and my 22" only has one handle Leave the lid in place.
Replace the 22" inch and continue cooking after messing with your fire.