Another gig coming up (Friday) and I'm not sure whether to put the clarinet on a separate mic (dynamic, cardioid) or not. Main reasons against: more to setup/takedown, and mainly more mixing for me! Main reasons for: better control over her volume and "place in the mix." Plus it make things a bit less congested around the other mic.
I welcome your opinions.
IMHO, two mics sounds very reasonable. I think it will come together easily, but if you don't like the results, you can quickly take the extra mic down.
If you get a chance before the gig, experiment with mic placement and EQ for the clarinet. I suggest having the clarinetist sit in a chair and play softly while you walk around. This will help you understand how the sound emanates from the instrument. I believe ideal mic placement may be above and slightly in front off the clarinet with the mic aimed slightly behind the bell. If this is too visually distracting consider aiming at a similar point on the instrument from below. I doubt aiming the mic straight down the barrel of the instrument will sound very nice, but feel free to try it.
In aiming the new mic, consider the need for each mic to pickup only it's intended sound source(s) while significantly rejecting all others. You may also need to modify how you aim the original mic to achieve best results. Consider moving it slightly closer to the guitar, vocal, mandolin sound sources as this will increase gain before feedback and help with rejection of the clarinet's sound.