Hi Martin, Condensing involves a lot of complicated calculations. What happens in the parts before and after this phrase? If there are notes either side without a rest inbetween, Dorico will treat that all as one long phrase (“phrase” for condensing = “anything between rests”). If at any point between rests for these instruments they have a different dynamic, rhythm, slur etc, then they can’t be condensed into a single voice and that’s the result you’re seeing - 2 voices.
You can tell Dorico to start a new phrase wherever you like by adding a condensing change at the position you want a “new phrase” to start, and ticking the condensing group you want to include. You don’t have to set any other options - just checking their box is enough to prompt Dorico to recalculate.
Thanks for the hint with the rests! I placed some rests before and after the phrase and then the problem was gone. The root cause was that Flute 2 was a not an active player and Flute 1 had no rest before Flute 2 joins. I could fix it with adding a condensing change as you proposed, changed the option for not active players and it worked.