I present to you: The Whiptail Lizard.
What is it you ask?
A lizard capable of parthenogenic reproduction in their female only species.
Why do you care?
Kangaskhan is basically the same. This also rules out the possibility of a male counterpart, but further backs up the possibility of not one, BUT TWO PREVOS.
My reasoning?
So we have the female only baby (shown in pouch), she evolves into the kickass teenager we see in mega form, then evolves one more time and goes through parthenogenesis while evolving to become the kangaskhan we all know and love, complete with carry on baby, She then goes into mega form, where her and her baby (now grown to the second stage middle version but technically not counting as a second pokemon because you don't own it, she does) kick ass side by side. The mega form then appears to wear off like it does with the other pokemon who have it, BUT WHAT REALLY HAPPENS is the teenager leaves to go off on her own and kangaskhan simply goes through parthenogenesis again to get another identical baby, AND YOU NEVER NOTICE THE DIFFERENCE. The teenager then evolves like her mom did and the cycle repeats.
How do you factor eggs into this?
Simple. Whiptails can also breed with other species, thus laying eggs. Kangaskhan can do the same. And there's no rule saying a pokemon can't lay and egg while it already is caring for a baby.
This further backs up GF's reasoning of not having the kangaskhan change during mega form because she doesn't, her baby simply grows.
See, now I really wish I worked at Gamefreak.