The AKG C516 system is a clip-on mic with short gooseneck. It's quite easy to get extra screw-on plates to go on your instruments, and then you can unclip the mic from one instrument and clip it on to the other.

Having said that, my usual spec for a festival stage, where I will be playing concertina, one row and two row melodeons and cello, is to ask for two instrument mics on short boom stands and use those for all the instruments (Left mic only for the cello)

In a ceilidh band I use an SM57 on a stand for the treble end and a Microvox for the bass, combined in a special battery box/mixer of my own design and sending a single feed to the mixer mic input.

I haven't found Microvox to be a problem with volume or feedback, but I stopped using them when I found I was getting distortion on my Oakwood box - either it's just too loud or the mics deteriorate with age, I not sure which.

Anahata