FSI: Form Union vs Assembly

Hi,

I have an FSI problem with multiple elastic plates floating on the surface of a fluid. When FORM UNION is used, the model fails when there occur collision between plates. I have tried FORM ASSEMBLY, with "IDENTITY BOUNDARY PAIRS" at fluid-solid interface. But I cannot select the boundary to apply the multiphysics FLUID STRUCTURE INTERACTION.

Does this means that FORM ASSEMBLY can only be applied for FLUID STRUCTURE INTERACTION, PAIR ?

Thanks and regards, Dharma