See, I don't think that would be the case. Those two girls have been doing their own rendition of 'Rain' for the past few years. Besides a few embellishments, much of what they did on TGS remains the same. Madonna sings melody and they just... do their thing. There would literally be no rehearsal needed. They've been doing all these years what they do best. Blending. All M would have to do is sing her part, which is what she does in the show.
Now... we just need the bridge and we'd all cream our mf pants and the show would end on a wet note, and then go to McDonalds.