Those videos are very useful. If you were just using say an X/Y pair on a bracket etc it could just come down to the height of the mics. Little higher to get the vocal more or lower for the banjo. That is if you were closer in. Do some test recordings.
If you miked it further away as
Mike suggests it would be up to the vocals to match the sound of the banjo. The banjo can be quite loud.
This might sound silly but you could try an X/Y thing tipped 90 degrees. So one mic is pointing slightly upward toward the voice and the other a little downward to the banjo. You would end up with the two things being just a little left and right of centre in the end. Could be good.