Edit: wait, I know one in Germany as well. It's expensive but between Frankfurt airport and another major city, there is a rail line but somehow the bus is significantly faster and they're using double deckers. The front row at the top was, n=3, always free for me (all four seats) so you get a quite unique experience, at least so far as my bus experiences in mainland western Europe go :)

cant they just put another entry door?

My local transit agency just stopped allowing rear door boarding, and that was one of the main reasons: https://waypointblog.com/capmetro-is-ending-rear-door-boardi...

People want to get to work on time after all. Entering and leaving the tram or bus through all door is a simple way to improve public transports reliability.

Often true. OTOH, the reduced footprint (vs. either 1 articulated or 2 normal busses) can speeds things up, especially on cramped and congested city streets. And if you're getting near gridlock-level congestion, the length of the articulated busses can be like dumping gravel into your system's gearbox.

> need a large harmonica to deal

I'd guess you mean "large accordion". Either way, the size & complexity of that joint section may be why the idea never went anywhere.

Double-deckers are cool, I’ll grant you that. And a very nice way to visit a city. But they are far, far less efficient in terms of capacity.