I don’t think you will have good results working directly on the segment, rather use it as a mask to execute grow from seeds as in the method explained here Suggestion: Quicker scissor tool and Seed growing to split a segment in two parts
You basically need to
- draw boundaries outside that segment ( to increase growfromseeds range (let’s call it GFS) )
- use masking options editable = inside that segment then draw the seeds in two separate new segments,
- then hide the original segment (so GFS does not see it as a seed) and execute GFS with editable = inside the hidden segment (GFS will then use it as a frame).