Not everyone may even know there's a rear staircase. I've boarded at the rear and used that staircase (but only after anyone has come down) but generally walk up the front and down the back, where it's easier to get off - albeit slower as those doors can take ages to open.
I like the dual staircase design.
The really important thing is the balance between stair and door locations. Even with forward and centre doors, a single forward stair can cause blockage to boarding if there is any number of passengers leaving the upper deck and particularly if there is standing on the lower deck. A single centre stair resolves this.
A rear stair doesn't really help flow compared to a centre stair, so I would suggest that the optimum two stair would be one forward (up) and one centre (down) with the doors to match (on/off).
Of course, if the second door is in the rear then forward and aft stairs could also work.
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However, what really matters is the on/off flow at each stop. A route which collects passengers from say a suburb and then deposits them over a few stops before a central terminus may not need a second door or staircase. However, if the route crosses the centre and is linked to another surburban route then the central part can have significant on/off at central stops.
Finally, and as I said on another thread, a central door with adjacent wheelchair/standing space can prove very efficient.