Stereo mics on piano is common, two mics on vocals is unusual. If you mix the two mics together to mono you get phase anomalies (hollow sound) when the singer moves slightly. The sound gets to one mic before the other and combines to cancel certain frequencies and add others.

An exception would be if you hard panned the two vocal mics and liked the effect of the voice shifting from ear to ear…

I think I would opt for one mic and more distance from the singer’s mouth. I’d try to point the null (stem) of the microphone at the piano so there is less of the piano in the vocal mic. If there isn’t a spot for it like that you could just do a stereo pair above the piano, catching the sound that comes out of the front of the piano where the lid flips up and see if the vocal/piano balance is there.

If it is a one off thing you may be able to rent condenser mics locally?