Carnatic (South Indian classical music), still sticks to violin as an accompanying instrument for vocals.
IMHO, the violin is the worst thing that has happened to carnatic music. carnatic music thrived for centuries, before the violin was introduced into it. the problem is simply that violinists tune the instrument in fifths, to whatever key the singer is singing in, and this makes makes for a scratchy, strident sound on most occasions. and when the violin is playing gamakaas (a very uniquely indian approach to modulation) in unison with the vocals, i think its just chaos, with nothing coming through clearly...
as regards hindustani music, the sarangi is still very much in vogue as the primary melodic accompanying instrument. it is true that the harmonium has certain limitations when it comes to exploring the entire scope of the music, but it makes for quite a nice pad to support the vocal and percussion. and if you've ever heard a brilliant harmonium player close up (and they are quite rare), you might revise your opinion. and there is a form of 'light' classical music called 'gazal' ('light' is a bit of a misnomer here, the music is actually pretty intense), in which the harmonium plays a key role, and does a damn good job of it.
to answer the OP, i mostly agree with d_fu... one close mic is sufficient in an amplified situation. however, if the music is not entirely classical, and you want a bigger and wider, if slightly unnatural sound, i recommend close micing using XY with a pair of SDCs. however, if the music is unamplified, contrary to d_fu, i believe that a minimalist non
close mic'd (waterlily styled) approach will work best.
FWIW, here is a sample of tabla and guitar (!) recorded with a pair of avenson sto2s placed between the instruments.