Can't say about accordion, Tim - no experience there! I use a single mic with both my octave mandolin (wot I ain't got no more!) and mandolin. Again, I put the mic about a foot away, but pointing at the soundhole. Sounds OK to me, but I haven't spent a lot of time 'faffing' with that one.

I don't think a small instrument like a mandolin needs 2 mics - not a complex sound like guitar. If you wanted to 'widen' the sound, I suppose you could always clone the mando track and pan/EQ them differently, but I've never bothered.

But I'm no expert matey, just an accountant and part-time amateur folkie who's messing around! :-)