The pictures explained a lot, the reason I posted the picture I did was because, like you say, I didn't understand why you were trying to do what you are. Now that I see the limitations, it makes sense to me. I'm not convinced that hooking them together is gaining you anything, 4' of head is 4' of head regardless of anything else. I don't think there are any synergies to be gained by having both pumps outputting into a common line, unless its pushing through a single check valve, which wouldn't make any sense at all.
