Trimming an offset line?

Hello everyone,

i was trying to find out how to trim a line which is actually a offset line of another one (in shaper mode). I can’t select the offset line when i activate the trim function. I’m using Salome 9.11.0. I can imagine that i somehow have to make the offset line kind of independent. But i could not find anything.

Any help is appreciated guys. Thanks in advance.