Humans are social creatures. It only makes sense that mutual stuffing is likely to yield greater consumption than going it alone. It's also likely to foster greater closeness between both parties.
Many years ago, I had some friends who used to smoke. When we hung out, if one of us lit a cigarette, the others nearly always followed. Out of that group, one girl and I began smoking more together. We made excuses to be around each other. There were numerous car rides, trips to the store, going together to buy cigarettes, etc.
The shared behavior brought us closer and we began dating. As the relationship progressed, simultaneously we were both smoking more. When we began living together it really ramped up to 1 1/2 to 2 packs per day for each of us.
Like with any other behaviors undertaken together, I have no doubt a mutual stuffing arrangement would generate similar increases in food consumption for both participants, particularly within a close relationship.
9 months