I did a B2B at BUD in May 2018. Now bear the time frame in mind - things may have changed since then. I documented my experience in my trip report, the specific post relating to the B2B can be found here:
https://billingham.net/post/17373648...ed-09-may-2018. I would imagine CWS will have some specific, more recent info about BUD, but back in 2018 it was a cinch for me. I woke the transfer staff up, so they are clearly not used to passengers doing transfers, but once I showed my incoming and outgoing boarding passes, all was fine.
Hope that helps!