I've seen pictures in various rags showing military personel with their mag's taped together with 100mph tape.  What do they use to separate the two mags from each other?  Are they still able to stow them in a mag pouch?

Thanks,
marm0t

Some people use small blocks of wood in between the mags.  However, the mags have to be staggered with the left mag about 1-2" higher than the right.  That way, when the left mag is inserted, the right mag is below the ejection port and dust cover of the rifle.  In this configuration, the GI mag pouch won't close, but one set of mags will fit inside.

However, you can also tape one of the mags upside down from the other, with no block.  Makes the setup ever longer.  

Personally, I've used the mag cinch and it throws off the rifle balance with full mags.  Plus, the free mag tends to eject cartridges while the weapon is firing.  I'd forget taped/cinched mags completely.

What he said.

Ditto to the taped mag idea, BUT when I was "playing around" with the idea, I used cut up pencils so all the sets of magazines I used would be uniformed in size.

I always used 100 mph tape with cardboard in between the two mags. Worked well for me in the field. Not efficient as was said if you want to store these mags in the "jungle mag" setup. Don't tape one mag upside down - these is the worse possible configuration. One magazine upside down causes that mag to lose rounds, jam up, or commonly get filled up with mud. Especially if you are doing anything remotely tactical in the outdoors. Magazine feed lips also get bent easily this way. Having the mags staggered side by side is a better system if you feel the need for two mags that readily available. I'd just go with a C-Mag if I felt the need for that kind of firepower or better yet a belt fed like the M249. If you have a semiauto AR, you should be good with just the 30rd mag in the AR with backup mags in pouches. All depends on the situation.