OK. Front magazine- Sweep hand back with index finger pointed down, withdraw the magazine from the double holder with index finger on the front of the magazine and pointed toward the follower. "Point" the magazine into the magazine well.
NOW. If you try the same maneuver with the REAR magazine, your finger will run right into the front pouch. SO, you withdraw the rear magazine by sweeping your left/weak hand from behind and forward with your index finger again contacting the front of the magazine just the same.
This was a well-established procedure at the time this type of magazine holder was in regular use. I got it from a graduate of Gunsite and saw it in use at local and sectional IPSC matches. It is my understanding that the equipment in vogue has now changed and I do not know (or care) if this procedure is still in use. I do know that it was a common and prefered practice in its time.