Connecting a Solar Panel and Power Supply in parallel, in order to power up a load
If you want the charging selection manually done, get a SPDT switch and and select between the two chargers.

Not knowing the voltages and currents involved makes the automated switchover suggestion tentative, but you can put OR'ing diodes on the outputs of both solar and USB charger output.  Highest voltage wins each diode prevents current from going back up the other charger (generally a bad thing).  

If you want less loss than a Shottkey type diode (usually 0.3 to 0.6V loss, depending on current and diode), you'd have to put together a MOSFET switch selector, that would get you nearly no loss on the charging paths.  

Make sure your components can handle the charging current, even a relatively low current 1-2A will heat up a small diode to an uncomfortable temperature, FETs are considerably better that way since they have very little voltage drop when properly biased.