Is it possible to force a phone to use USB for data but not charge from it?
Considering usb C can send power in both directions, I'm assuming you can set the phone to power an external device using in phone settings, and this way you would avoid it pulling power from your source. 

A side note though : 2A is quite a number, unless you're running rpi hot or powering something else my educated guess is that it should be enough for both.