Went thro BER the other day, (connecting, but not on a B2B). Schengen, non-Schengen arrivals and departures all on different floors of the same terminal. I think it'd be reasonably straightforward to do a B2B if you have already checked in for the return flight. You walk along the arrivals corridor, go through connections security (I apologised to them for disturbing their chat - I think I may have been the only passenger that day! ), up two floors in the lift/stairs, and back along the departures concourse to the departure gate. The terminal's quite long, so - depending on where you arrive - it could take you a good 10 minutes to walk gate-to-gate, but it's do-able.

If it's a long connection, note the lounge is accessed from the Schengen area - so you need to enter Germany (either via arrivals, or going thro the passport control after transfer security) to get there! I was looking for the lounge, and ended up asking the team boarding the return BA flight where it was - hence my confidence you could do a B2B if you wanted to...