Southwest has always done front and rear doors at BUR, since there are no jetways, but at airports with jetways it's always front only. I haven't seen them do A via the front and B via the back as noted in the linked thread. My typical procedure as a high-number A is to go to the back stairway and then up the aisle to the front of the aircraft. Usually I can be in the front few rows before the first group aboard have stowed their carryons in the overhead. Flying into BUR I sit near the back and exit via the rear stairs-- most people reflexively move forward because so few flights exit via the back.

But as WHBM points out-- if you look out of a typical terminal and try to see how you're going to maneuver a second jetway to each plane (getting around the wings of that plane and the adjacent ones) there are serious space constraints. It would make more sense on widebodies to use left and right doors, rather than trying to get a second jetway around to the back.