I'm familiar with using figure 8 mics to record a singing guitarist,
positioning so that each of the guitar & voice is in the null of one of the
mics. A different technique occurred to me that I can't test easily as I
have no singing guitarists handy. That's micing each with a cardioid
pointing at the source, then adding a fig 8 mic pointed so that one side
gets the vocals and the other gets the guitar. By combining this with the
cardioid tracks and flipping the polarity, in theory you should be able to
cancel out one or the other. Now I know this assumes the cardioids and the
fig 8 are the same distance from the source which mightn't be practical (or
the best position for other reasons), but is it a technique worth
investigating and do people use this in practice?