I would suggest trying the XY pattern and experimenting, on your own with the angle of the microphones, as well as the distance from the vocalist's mouth. I think if you get the mics close enough to the singer's mouth you will start to have a more balanced mix only using two channels.
When I started my own experiments with recording my voice and acoustics guitar I began with a 90 degree angle but after a while, at the expense of stereo reproduction, I decreased the angle to say 60 degrees, the better to address the singer to mic (but with a poorer stereo image).
If you are open-minded you can also try a spaced pair, with one mic picking up the singer directly, and the second mic behind the singer, essentially picking up the room including the cello. Without looking too far, I remembered that the Sound Pure dealership has several videos where they demonstrate mic positioning. Here's one:
https://www.youtube.com/watch?v=CnwklTUWPsY
I don't know exactly what your tastes are but mine lean toward trying to pick up a choral or acoustic interaction between the singer and the instrument. If you close-mic each of them I can't see how you will get it. My own belief is that there is a detectable acoustic artifact that happens through close-miking and then mixing the separate tracks. If you can record a decent stereo mix, and and have the talent adjust the relative balance of the voice and cello it will be great.
Ultimately I discovered a dual monaural mic -- the Josephson C700A -- and I am happy with it (but I also have a few other options).