Record both singers at once, off-axis.

Yes, you can pull off a duet like “I Didn’t Want to Miss You” with a single mic, but you have to treat it like a live performance. No fixing it later.

Put the mic in the middle, have the singers stand facing each other at about 45 degrees off the capsule. That way each voice hits the diaphragm at a slightly different angle, which reduces comb filtering and gives you a natural stereo image. Cardioid pattern works fine; figure-8 is better if you have it, because the null to the side kills any room reflection from the opposite singer.

The hard part is balance. Each singer has to control their own volume in real time — you can’t adjust individual levels afterward. So do a test take, move them closer or farther, adjust angles. A couple inches can change everything.