Wouldn’t it make sense to test it and decide? It’s like recording other kinds of acoustic instruments where there are certainly conventions that one can follow but ultimately different mic positions would result in different characteristics that works for certain context in music.
If I had one of these I would be interested to listen with my ears how the different parts sound upclose (for close miking) as well as how the room responds to it (for distant miking). I would also try to capture the aspects of it which I find most enjoyable in comparison to just pluging my synth straigth into a regular speaker.
For phase issues you can try to get the front and back mics equidistant (in relation to the Onde) and reverse the phase in post like you would when miking a snare drum from top and bottom for example. Phase issues will have to be confirmed in post of course but two mics won’t be as difficult to get into alignment.
I wouldn’t be too precious with how I record such a thing as the point of it seems be to introduce some artifacts and non-linearities into a signal that can easily become sterile otherwise.