Nothing new but I've been playing with something old. Several years ago I bought a Choate Magazine Connector. Instead of using it on my 30-round mags I modified it for use with 20-round mags. My Colt AR came with 2 20-round mags with spacers to block them off at 5 rounds. The Colt floorplates were made just for these mags that had flat springs in the bottom held on with rivets. Of course I removed the rivets and spacers, but then there were no spring latches to lock the floorplates in place. Brownells to the rescue! Two of their G.I. style floorplates and I was good to go.
But the magazine connector was way too long to clamp the 20-round mags together and be able to use them. I cut the top and bottom off of the steel plates so there's just a hairline of metal between the edges and the reinforcement ribs. I cut down the plastic filler block so it's now flat and just slightly below the edges of the steel plates. I clamped my mags together with the edge of the front plate sticking ever so slightly below the edge of the mags. That gives me 1/8" of space between the top of the rear plate and the mag well.
It's easy to eject the empty mag, move it over an inch and insert the other one. When the left mag is in with the dust cover previously opened, the right mag makes contact with the part of the cover containing the spring and detent. It should keep them from rattling against each other. I can keep the magazines blocked at 5 rounds with a quick reload if I start hunting again, or take the spacers out and have 20 rounds in each. I have G.I. magazine pouches that I carefully removed the grenade pockets from. 2 mags clamped together fit great in the pouches made for 3 mags when the dividers are cut out. If you get cheap used military surplus mag pouches, someone probably cut them out already. But the 20-round mags go down so far in 30-round mag pouches that they're hard to reach. That's where my sponge rubber stress ball comes in. I keep it in the bottom of one mag pouch to avoid the stress of fishing out my mags. It weighs next to nothing and makes a good spacer.