Relevant to my interests:
As you can see, you with the pictured adapter you can couple 2 Magpul PMAGs base to base to get a maximum of 60 rounds (2 30 round magazines). You get a new longer spring of course with the coupler.
You can get the coupler from the source California Competition Works for $45 – HERE
Ridiculous looking, but awesome. It would be hilarious if someone came up with an coupler for the 100 round surefire aluminum magazines.