Spoiler: Video solution https://drive.google.com/file/d/1bj8fS-4prKVgT-CQDeg4PdTHVOPHOqgY/view?usp=drivesdk
Okay, I played it through. You have to click on the stairs directly at the bridge, otherwise B will get stuck. Then it works.
Played it twice and everything worked smoothly now. One of the stars is embarrassingly easy and pointless though.
Movable is still loose but enough worthy to walk over. Very well repaired! I liked a you removed side facing Stair with Fence, that helped B to not to fall. Nice level.
That was a fun and exciting Cooperative level and a quick one. I like the bridge that connected The Two Towers