Then there are all different variations of redundancy. Completely independent Main 1 and Main 2 process bus network and components or a redundant communication network like HSR where all main 1 and main 2 components are connected to. Both approaches have advantages and disadvantages. The second approach allows the main 1 protection device to still operate and use the data of main 2 merging unit if main 1 merging unit fails. This may be higher availability. On the other side it is more complex and the independence of main 1 and main 2 protection is lost, since they share a common redundant network. If the first solution fulfills your reliability requirements, why would you go to solution 2?