Why have I never heard of a Combo Mortage?

[Evan Parker]

I was talking with a friend today and he told me about the possibility of getting a conventional loan along with a second morgtage to help "offset" the initial down payment to technically avoid PMI and the 20% down requirement.

When I read more about it I got very excited! But why isn't this talked about more? 

Do any of you have experiences with a Combo Mortgage? If it is what I expect it to be, I would be looking into getting a combo mortgage for my wife and I on a home in ATL as a primary residence and use the additional money that we'd otherwise spend to meet the 20% down payment as more investment capital, instead of grinding it out to save that amount of cash and emergency fund back up to par for other purchases. 

Also, if one does go with a Combo Mortgage (I'm calling it that because I don't know if there is some other official name it goes by), how do HELOCs work with that? Because technically I wouldn't have a high amount of equity in the property.