It is a bit vague what you want to do. Do you want the displacements/forces to be added, subtracted…? What data is mapped from where to where? What happens with the forces/displacements when they are communicated in the opposite direction? What data is mapped from which interface to which other interface?
I worked on an application with similar setup that was discussed here and here. This ultimately led to the implementation of the “Multiple mappings to and from a mesh” feature by the preCICE team (thanks again for that). This feature might be interesting for you. You could also (artificially) create two meshes for the solid solver, e.g., Solid-Mesh-Up and Solid-Mesh-Down.
Edit Just as an addition. You could also merge your fluid meshes into one Fluid-Mesh instead of have a Fluid-Mesh-Up and Fluid-Mesh-Down. I assume that is not possible for your application/setup, but it is hard to tell what you want to achieve.