You posting Molly and Jack together like that made me think of a sibset I know of: Lucy, Rosie, Molly and Jack. The only mn I know from the set is MollyKate, which is of course another nickname! I have to admit, it's cute, but I think MaryKatherine is even nicer!
Oh, I know two kids named Molly and Jack too. I remember my dad grumbling about "not proper names" when they were born. That was probably right at the beginning of the pet-forms-for-full-names fashion, in the 90s.