Hi,
And tx for the time you spend with this "project", but the split should work, yet, if there would be no bug at this place of F360.
I should get a body, with a little missing near the boundary of the body 1, but the split should works, yet (to see the body 1, put the timeline after the form1, the 1st item).
Here the dual version of this construction, which is closer than what I finally want to get (body 3 splitted with body 6 and not body 5).
There is no extrude construction, with the split tool.
So, that should work, but it don't.
Those projects are just isolating what I guessed, about such kind of splitting.
For me, it's really a bug of F360, and really a huge waste of time, when an user is facing such problems.
Btw, I noticed another problem, preparing this file.
In the sketch 1, the curves get with the boundary of bodies 1 and 2 had 2 copies, because I surely clicked 2 times, with the Project / Include -> Include 3D geometry of the sketch menu.
I think that the program should say : "Do you really want 2 items in the sketch for this curve ?", because I bet that it can trigger some bugs and others merrymakings, further, in the building of the project.
Tx for your lighting
PS : the good file is dual split2. The surfaces weren't stiched in the first file.