A key technique when joining two flags together in one hand (marry) and then being able to later separate them back into individual flags (divorce?) is to place your index finger between the two flags.  You can still move them as a single flag with one hand, but it is much easier to break them apart while in movement.
 
Practice doing a simple double down and then "marry right" so that both flags move to your right hand.  Perform a single hand down and then break them back into two flags doing the double down.  Now do the same thing with a "marry left".