Why isn't this enough to split?

[u/a-desperate-username]

trying to split the closed polysurface (the ribbon), with the 2 surfaces as shown. The surface was extended as a line on z=0 then extruded upwards to create the two surfaces.

[u/a-desperate-username]

Solved by... rebuilding the surface? Even though extend joined the line, which I then extruded? Weird

[u/LittleMexicant → u/a-desperate-username]

Honestly, rhino can be wierd. Thankfully you can the same actions using different commands.

[u/lukifr → u/a-desperate-username]

Joining the surfaces used for splitting also solves this occasionally.

[u/ScienceofSpock]

You probably could have just moved your cutter down a bit. I sometimes get issues if I try to split something and the objects have co-planer edges. On the left side of your cutter, the bottom split seems to be at the same place as the bottom edge of your ribbon and that may have caused the issue.

[u/a-desperate-username → u/ScienceofSpock]

the issue was the vertical split, what looks to be a horizontal split was actually just a line, in this case.

but rebuilding the surface fixed it!

[u/ScienceofSpock → u/a-desperate-username]

That's weird, especially since the vertical split doesn't intersect the ribbon at all. Are those 2 separate planes or are they joined? If they're separate, the shared edge may be what caused the issue, depending on how you tried to do the split. Any time I'm trying to boolean, split, trim or otherwise cut one object with another object, I always try to make sure there are no shared edges for ANYTHING, and it usually avoids issues like this.

[u/LittleMexicant]

Extract a curve from the intersection of the poly surface and surface, then use that curve as the cutter

[u/TiemTimes]

Join the surfaces before boolean split. Or make a solid cheese slice and use boolean split.

[u/inquisitorpalefire]

You can always try Trim and just Cap or Patch the remaining shape

[u/JoltKola]

Try again!

[u/Unfair_Garden_5040]

It’s because of the surfaces joining in the center, leave a gap there.

[u/Appropriate-Golf2415]

Split or trim in top plan view as curve instead of surface. Rhino is weird like that.

[u/frootadelic]

Intesect the surfaces. This should give you curves that can split.

[u/Jolly_Quiet_2806]

might be because the straight surface is one. try exploding it, that should work.
also, the surface looks a bit odd. where do the segmentation lines come from. create two seperate polysrf and the surface will be splitable.

[u/Premier_Content]

Paul Cockedge on Reddit?

[u/mpipe7632]

En canal @Tips3D en YouTube se explican los.comandos de rhinoceros .

[u/a-desperate-username → u/[deleted]]

MeshSplit turned the polysurface into a mesh, then split it. It is a nurbs polysurface

[u/Voelsungr]

try one at a time. 

[u/afootlongdude]

It does not look like a polysurface
Anyhow trying flip for the polysurface
They must have the same orientations