This is actually an assertion deep in the partitioning package.
As @Makis said, this should be an error with a user-friendly message.
What this means is that when running in parallel, there is no overlap between the meshes of both solvers.
If it still fails running both solvers in on a single core each, then this is a strong indicator that the meshes are not describing the same geometry. The error message will be different in that case
Otherwise, it could also be that the interface meshes are super coarse, which could lead to problems.