Merge surface

I made a vacum forming tube form, by making half of it, and then mirror for the second half.
now I want to merge the surfaces, each 2 surfaces to become 1, not a polysurface.
How should I do it? Mergesurf doesn’t work…
Attached is a picture of my file


Just important thing to say - when I do join command, it all becomes a closed surface with no naked edges !!!

and this is the message that I get : “Can’t merge trimmed edges of surfaces”

Hi Rachel- Surfaces must be untrimmed in order to merge- it is unlikely that all of the surfaces you show can be merged. Why do you want a single surface as opposed to a polysurface?


Hi Pascal, thank you very much for your answer. I just thought that there is an option similar to “merge two coplanar surfaces” for not planar surfaces. Is there? thanks, Rachel