What are working stereo USB microphone options for the iPad? It needs to be usable as an audio input into recorder apps? Presumably plugged into the iPad through the USB camera connection adapter.
The manufacturer confirms that this is possible, and has detailed directions for setting it up (including a video) on this page.
Note that you will need a powered USB hub.
I think what you desire most is to purchase one good USB microphone which can be used with multiple computing devices besides just the iPad.
Here is another strategy: Get an iPad-to-audio interface that you can use to connect to conventional microphones.
IK Multimedia, Apogee and Line 6 also make similar products but they are designed for guitar input, not microphone input. Trying to get one of these to work with a microphone would be tricky due to differing impedance levels and preamplifier requirements.