There is indeed a technical limitation of preCICE and your current solution is the recommended way to go. There are also some conceptual questions however.
The last two make a lot of sense, but the first two are not well-defined. Which values do want on interfaceB, should the one write mapping overwrite the other one or do you want the sum? The latter makes probably sense if you have two conservative mappings, but what should happen for two consistent mappings or if a consistent and a conservative mapping are mixed?
In your case which behavior would you want?