Indeed, using MixedElement implies the function spaces to be defined on the same (sub)mesh.
We are working on a new version of MixedFunctionSpace allowing to define product of function spaces from various submeshes, which can eventually have different topological dimensions.
This feature will be available soon, in a next release.
In the meanwhile you can give a try to the development version using the Docker image : https://hub.docker.com/r/ceciledc/fenics_mixed_dimensional
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