Clipped STANAG magazines work as a single 60 round magazine as opposed to two different STANAG magazines, they should need to be swapped (one for the other) after 30 rounds have been fired, or removed until they can be used that way. They also should be able to be taken apart to receive the two 30 round STANAG magazines

Clipped STANAG magazines already have an advantage over single is that they are literally 2 magazines taking the inventory space of a single magazine. If Double-Beta Drum magazines are ever introduced (which i'm guessing is something you're probably planning on at some point) they'd have to take up a minimum of 4 slots, and nobody would use them, because with these "magical 60-round magazines" only taking up 2 slots, their 4 slot total can hold 120 rounds, at the cost of a single reload.

They are also typically too easy to find (I've found like 100 of them, over the course of my play time) where as the single STANAG magazines seem harder to find (I've only seen them a couple of times so far).