Switching input voltage, with two 3,3V inputs
There are a few ways to do this. 

Personally, I think the easiest way is to get regulators that have a shutdown pin. With the battery input, you can have a pull-up resistor on the SHUTDOWN pin of the regulator. Have an N-channel MOSFET there aswell. When the USB is active, it toggles the MOSFET and pulls the shutdown pin low, which will effectively disable that regulator, then you can use the USB power. 

Here is a very simplified version of this. It has all the information you need, but will still need a bit of work to make it applicable for your needs.







simulate this circuit &ndash; Schematic created using CircuitLab

EDIT

If you have a regulator with and active high disable pin, (as the example by OP) the above schematic could be edited as follows:

EDIT 2

Schematic fixed







simulate this circuit