Hi Jozef,

> it is not possible, moi does nothing, I still have 2 separate solids... what to search for?

Well, I'd probably just take a general look for problems, like self-intersections, slivers, similar to what I looked for in your other case. See if using ShrinkTrimmedSrf gives different behavior or not.

Can you post the 3DM model file (or send it to me at moi@moi3d.com if you want to keep it private) and I'll take a look and see if anything stands out.

If your objects mate up along the cut line, it should be possible to delete the faces along that line and then with 2 pieces that have common open edges select them and use Edit > Join to glue them together rather than boolean union. Join only tries to glue unattached edges to each other, while boolean union also tries to intersect the objects and see if any pieces need to be removed, so it's a much more complex operation than join.

If your edits were right adjacent to the connecting faces with new pieces partially overlapping it may be difficult for the intersector to figure out how to cleanly divide it into separate chunks.

- Michael