I like the concept of being able to split a 20 round 7.62 into two 10 round 7.62. So if you have a bolt action you can still pick up 20 rounders and use them. Another thing that is cool is if you have 2 7.62 mags one with 9 round left and another with 2 rounds left you can combine them into one mag with 11 rounds. I think it would be neat but will take a lot of getting used to.
I am assuming, and I may be wrong, but to do that you will have to empty one (with some possible slow animation) and then use the loose bullets to load the other (another slow animation). I don't think it would be the right click, select combine from drop down, instant new mag.
and to take a 20 and turn it into 2 10's, you would have had to of found two empty 10 round mags somewhere first before you can load them up.
plus that is a lot of empty mags you are going to be carrying around while looking for single rounds to load up in there. I would assume they take up the same volumetric space empty or full, so bag space management is going to be a lot more of a bitch.