Two power sources, auto-switch?
Yes, this is simple.  It can be done passively with two diodes.  At these low voltages, they can be Schottky diodes, thereby dropping less voltage and being more efficient.

Put a diode from each source to a internal power node.  When 12 V is present, the internal power node will be at just under 12 V.  The diode to the 7.2 V source is therefore reverse biased, essentially disconnecting the 7.2 V source.  When the 12 V source goes away, the internal power node will be at just under the 7.2 V.

A switching power supply can deal with this variation in source voltage and still produce your final intended 5 V reasonably efficiently regardless of which source is providing the power.