For any transit system using DD for local route, you "NEED" 2 door and have seen 3 on some systems in Europe. Hell!!, I even saw 3-4 doors on 40' buses and up to 5 on artic's
From my experience using both single and twin doors in Europe, your idle time can be a few seconds more than normal, depending on the load and stop and that can add up based on the number of stops.
Wasn't sure about using DD on local routes before going to Europe and nothing wrong using them after ridding them. I used both levels as well watch riders using the upper level and saw no real issue at all. Using DD is a lot better than artic's in the winter, let alone less room for bus terminal. Can seat more than artic. It becomes a learning curb at first for everyone, but it comes secondhand after time using the upper level a few time.
All DD I rode met AODA standards. Some DD had 2 stairs.