I know this wasn't what you are asking for exactly...but I thought I should recommend that you look into the Orbit Double Helix if you hadn't already.
I'm not sure about the weight limit on it, but it converts really easily to a single. My cousin has the Donkey for her twins and I think the Orbit is easier to conver to a single. I also know that she sometimes had trouble fitting through doors when she had the car seats installed (she had twins though so in her case it was two car seats at the same time). It's not as bad anymore now that the twins are bigger and they just use the regular seat, but I know that there are still situations when she doesn't fit through a non-standard doorway.
Orbit also has a skateboard attachment so that your older one can ride along when she is tired (although I think Bugaboo has something similar).
For infants I think Orbit is far superior since it comes with the infant car seat which has good safety ratings and is sooo convenient for getting LO in and out of the car. I'm very petite and I had no problems getting DD in and out of my SUV since I could just swivel the seat towards me.