USB microphones are seen by the devices they're connected to as individual sound sources, and most devices will only handle one sound source at a time. It's doubtful what you're proposing here would work. You'd be better off trying a USB mixer with two analog microphones connected to it, and then connecting the mixer using a proper adapter or cable to the iPad.
Connecting to USB microphones to a computer usually requires massive workarounds with varying results. It seems unlikely it'd work with an iPad.