To put mags together; install one in the rifle. Rotate the other one opposite- ammo will face downward, and forward. Use cloth tape; less residue than duct tape. Tape 'em together. Leave 1/2" clearance from the bottom of the receiver to the bottom of the inverted mag.
I'm 41, and damn near too old to do this.... (Homage to the other fossils who are pointing out mall ninja status)
I will tell you of the drawbacks in the real world doing this: Your flipped mag will be open to the dust and debris of the real world, unless you get magazine covers from Troy ind. for STANAG style mags, or you use Magpul Pmags and keep their cover on. One of 3 things will happen: You will be depending on the proper function of the rifle- let's say your life depends on it (training is real life) You will try to stuff that mag into the well with the cover on................
or, you will spend what seems like 2 hours getting that otherwise easy to remove Pmag cover off, so you can stuff it home. Last scenario, you will have attempted steps #1 and 2 previously. For mistake #3, you will run the flip mags without a cover. You will inevitably drop to the ground to fire. You will find that the flipped mag setup leaves the rifle propped up like 2 stories high, and you are having trouble shooting. You will also find that, during your haste, you dropped into a pile of fresh cow manure, and the magazine that was exposed on the bottom is now plugged with said manure. You will flip said mag into the gun, and the rifle will shoot like ****. ............
Wear Molle pouches brother.