Multiple doors would make a huge difference on the busway, as well as on other busy corridors, but I doubt they'll ever be ordered as there are too many places they wouldn't work. Drive-in-reverse-out bus stations seem to be favoured nowadays, and they have their advantages, but they also have some pretty major shortcomings and one of those is that they restrict vehicles using them to front doors only. Also plenty of roadside stops with shelters or other street furniture that would block the doors.
That said, street furniture can be moved and has to be replaced periodically anyway, so ensuring it is compatible would be a prudent decision in case it is desirable in the future. And the DI-RO bus stations in Manchester mostly (if not entirely) have a separate alighting stand which is outside the main building and could accommodate multiple doors. At a bus station, the ability to board through doors other than the front one isn't so important because, as long as the driver starts letting passengers on in good time, slow boarding won't delay the bus. (In fact, boarding through the front door only at all times might be desirable, for revenue protection reasons)
