The first thing to be aware of is, if you are using two power sources, they will not have exactly the same voltage, so power will flow from one to the other in ways that you don't intend...with potentially disasterous consequences. This is, by the way, why hubs are not supposed to backpower, and good quality ones don't. For any hub, you should *know* whether or not it will and adjust your practices accordingly.
It is possible to backpower *some* models of Pi starting with booting the Pi. Ones that can do that are the A+, Pi Zero, and Model B Pis. Ones for which this won't work (because the power won't be fed back from the USB ports to the rest of the board until the LAN chip is up and running) are the Pi3B, Pi2B, Pi2B2, and--if I'm not mistaken--the B+. Also note that it is going to be an exceptional powered hub that will provide enough backpower current to fully power a Pi2B or Pi3B.