That's the best option - an extended rail or riser on the AR, and good QR rings on the scope, but you might have a trade-off to get the right height on both.
You'll need 1.4" or so centerline on the AR, as skipsan says, and most rails are 0.5" (though a few are taller, like the RRA at 0.67"). You'd want to use rings high enough to get you to 1.4" on the riser, and they'd likely be a bit high on the other rifle, depending on the scope, but it'll work.
The other problem is keeping track of your zeros for each rifle. You'd zero it on one, then swap it to the other and re-zero, making a note of how many clicks and which direction. Once you had that, you'd change it that much each time you swapped between rifles, then send a few downrange to make sure it's good. After a few tries, you'd have a pretty good idea of how stable it is.
I used to do this on a few rifles, but honestly, it's a pain in the butt, and I've ended up buying more scopes that cost less to help avoid it. YMMV, as always!