When I'm the guard of the
rear set I open the rear doors with the local switch and stand there checking tickets as passengers board - any for the
front set are politely directed to the correct train. I then release the remaining doors when the
road comes off for the front train. I rarely experience any problems with this practice.
I like my trains loaded early to sort out any luggage and other problems prior to departure - three minutes is not enough!
As an aside, in general, Glasgow Central seems to keep the doors open with multiple trains on the same platform and use mobile front/rear train boards to differentiate. This was tried at the Waverley but the through winds (sometimes from Siberia) tended to blow the boards away!