You could use a coupling constraint for this purpose. It’s not yet supported in GUI but can be defined manually with proper keywords: RBE3 integration in PrepoMax (Kinematic & Distributing Coupling)
However, regardless of the type of constraint, you may encounter similar issues if BCs interfere with constraints. Thus, you may have to change the modeling approach and the way load is applied to the structure. Details depend on how it’s supposed to be loaded in real life.