…how is it done? I’m used to working with boolean operations such as union, punch etc. where forms are created by punching holes into objects with other objects or combining them.
Glyphs probably uses some similar commands but what are they and where can they be found?
Filter > Remove Overlap
This seems to work only half of the time. Sometimes the outcome is as wanted, sometimes the forms are intersected, creating several non-intersecting but separate forms.
This is probably a bug?
Paths that travel in the same direction they merge, paths traveling in different direction intersect.
Okay. There must be some logical reason for this but in contrast to logic of other vector-based programs that’s some weird behaviour. So how does one merge two crossing forms? Is it not supported at all? 
in font, the path direction is important (unlike in other vector drawing apps). Counterclockwise paths are considered “black”, clockwise paths “white”. An “O” is build from one ccw path and one cw path that cuts a hole in the outer shape.
So in your case, you need to reverse the path direction for on of the two paths. Right click one of the nodes and choose “Reverse Path” from the context menu.
Okay. I feel stupid now. DTF’s advice seems very clear all of a sudden.
Works fine now.
Thank you both!