Can't intersect this part (or trim surface) | file provided

Hello everyone,
i have a problem with a specific surface (see picture below).
How could I trim/remove this surface? Intersecting these two surfaces gives me 2 single lines / not a closed line.
Or is there a better solution? I hope the picture shows my intend.
Thank you for helping!
Using Rhino8


Cant Intersect.3dm (293.5 KB)

An incomplete intersection curve is a pretty typical Failure mode in Rhino’s ability to create an intersection between two surfaces that have an area where both surfaces are tangent to each other (they have the same normal directions).

What you can do to get a more complete intersection is add a few knots to each surface in the area where the intersection fails and then you get close to a complete intersection. To make the intersection go all the way just add a line at the end.

Once you have a complete intersection you can copy it to the clipboard and then undo back to before you added knots and paste the curve back in. You can then trim and join the two surfaces:
Can_Intersect.3dm (369.0 KB)

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