@Bo-Peeps You’ve figured it out!!! I’m convinced of it. That is the ONLY way Neal’s random, bizarre death and his whole “you showed me what it means to sacrifice” makes ANY sense at all.
You are totally right, they only ever needed one heart to survive. If Neal “made” Rumple sacrifice his heart for the curse than Rumple sacrificed the thing he loved most and Rumple could cast the curse (or I guess technically even Neal could have cast it). Neal was going to die anyway because the way he brought Rumple back required his life in exchange -so it is kind of like the EF version of organ donation.