I had issues with the width and to some extent the seating positions too but my children were the exact same age as yours when I had it so didn't fully experience how it would be for two children sitting up as oppose to always having the lower one lying flat. With the wheelguards on it is comfortably over 70cm wide, although it doesn't take up as much overall space as a side by side it's so much wider than a lot of other tandems that I did still run into issues.
I found it really easy to push but we have no hills at all so it was possible to push one handed whenever necessary, which definitely isn't true of all tandems.
For the australian market britax/steelcraft did actually make a compact of their version of the b dual, which incidentally already had different seats (bucket, not lie flat) called the strider compact. I asked britax if they had any plans to bring that here, their reply was something along the lines of we don't think people people will want it, with the smaller size comes smaller seats. Tbh I don't know if that is true.
After the b dual I had a smyths toys dimples duo, which was great with baby lying flat on top parent facing and toddler underneath ff. Not the same quality as the b dual though. 1cm narrower, which for some reason made the difference between scraping into my house/playgroup/everyplaceiseemtogothatsnarrow and getting in easily.
What else are you considering?