FWIW, I'm NOT a "very large woman" and I still have used 2 gowns per the staff's instructions.
What I would THINK would work is a standard over-the-head gown pattern, but cut 2 fronts and 1 back...instead of sewing the front to the back, finish each edge with a hem on the "first front" and then the opposite side of the "second front".
having 3 sleeves may defeat the purpose of the 2-hospital gown thing, so perhaps snaps or ties might be the answer?