Another option is run your omni's, essentially in the middle of their setup, . Midway between the monitors left to right (or if that's too close to the drums, favor the keys side), and backed up far enough onto the stage to pick up the monitors. They need to hear their vocals, and you'll get 'em too from the monitors. MMW, albeit sans vocals, have done many onstage tapes that way, at least back in the day, with Earthworks omnis (they ran the mics facing left and right, with the capsule on one mic lined up with the back of the other mic)
C-----------
-----------C (with C being the capsule)