How to combine two grids into a single grid

[dokken]

This is your second post today that does not follow the guidelines in

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and does not explain what version of fenics you are using, and does not provide a minimal attempt to solve the problem.

Please also check the forum for similar posts:

How to combine meshes in dophinx?

You cant define two unrelated meshes. I would suggest you use MeshView to create the 1D mesh as part of the 2D mesh, and then use the assemble_mixed command. See for instance which contains links to relevant papers and changes in API