No problem - use the right index finger to actuate the mag release lever while the left hand is bringing a fresh magazine up. Most of my empties drop free when I hit the lever, but the left hand with the fresh mag can knock it forward and out very easily if they don't drop on their own. New mag goes in and rocks back to seat with the left hand, all while keeping the gun on target. It works almost as quickly as an IDPA reload.
Of course - if you want to stow your empty mags that works the same way, just stow the empty with the left hand and then continue as above.