Yes, Mechanical participant only supports single System Coupling Region. Please create corresponding surface in Fluent to ensure good mapping and define multiple coupling interfaces in system coupling module.
And regarding solution divergence, Check whether the force sent to Mechanical was too large. This can happen if Fluent didn’t converge, or if there is an FSI instability due to similar densities between solid and fluid.
Using under-relaxation on the data transfers can help here. This will make the solution more stable at the expense of more COUPLING ITERATIONS to reach convergence. Note that some cases can remain unstable even when under-relaxation is used; to stabilize these types of cases Solution Stabilization is available in Fluent in the Dynamic Mesh settings on the Solver Options panel.
Hope this helps!