Parallel USB Power

General question. I'm developing a project that is a standalone device but also can be a shield and plug into an Arduino Uno R3. My project powers itself and provides power to the 5V and 3.3V pins that the Arduino connects to. Generally this is frowned upon however I've engineered nearly all the power cases such that the Uno R3 will be protected. Basically the one that gets most people is powering the Arduino 5V externally however my project ALSO uses the Vin pin for it's own 5V regulator so I guarantee the Arduinos reverse protection MOSFET to protect me in that case.

However, the one situation I can't eliminate seems to be two seperat USB power sources. My project/shield obtains USB power and provides it to the 5V pin (ideal diode/MUX power switching w/ on-board regulator).

IF my project is plugged into USB AND the Arduino is plugged into USB then the two USB power sources are essentially in parallel.

1) I assume this is likely OK (though not USB compliant) when both USB ports are the same computer 2) I assume it's probably a bad idea to use two USB power sources that are different i.e. two different computers or one computer and one USB wall adapter.

Are my assumptions correct? Thoughts?

Again I'm NOT asking about whether powering the UNO from the 5V pin is OK. I've addressed that issue, this is purely USB power in parallel from Uno to my project.

I've checked the forum for similar topics but nothing seems to specifically answer my thoughts.

Generally this is frowned upon

For the obvious reason that those are output pins.

Bring power from each in via series low Vf schottky diodes so one source can't be backfed from the other source. Example: http://www.digikey.com/product-detail/en/1N5817-TP/1N5817-TPCT-ND/950586