A bit confusing. IF you could replace a single BFP with two 3/4" ones, then the original one could not have been much more than 1", and a 1", or even 1 1/4", BFP would NEVER overwhelm a 4" sewer line, and would absolutely NOT require a 10" sewer. The assumption that "only one of them would backflow at a time" would ONLY be valid if they were on two different systems, (which could not be the case if the two valves are replacing a single one)), otherwise the same conditions which would cause one to backflow would also operate the second one at the same time. The other question would be, "Where is all of this water coming from", unless you have two separate systems tied together. Otherwise the only "backflow" should be whatever water is contained in the downstream, usually closed, system. Did you leave out some critical information?