My first suggestion would be to run a SBD feed into the third channel. Lacking SBD access, give a three microphone array a try. Its fun! Here are some guidelines for doing that:
1) You can use three mics that all have the same pickup pattern, or two mics with the same pattern and a third with a different pattern. The Left/Right pair should always both have the same pattern.
2) The third mic should be placed in the middle and can be faced forward towards the stage, or backwards away from the stage. Both options can make for a useful addition.
3) Most tapers will feel more comfortable with a
forward facing center mic
In regards to that:
a) In very basic terms, think of a three mic stereo setup which uses identical microphones as being
two adjacent stereo pairs that share the center microphone. This is overly simplistic but conceptually makes for a good start with regards to the following..
a) If using 3 mics of the same pattern,
space or angle the Left/Right pair twice as far apart as you would when using them as a single pair on its own.
b) You can use a combination of additional angle plus additional spacing to reduce the
twice as much requirement somewhat. If you can't (or don't dare to) space or angle twice as much, definitely use a combination of both.
A good general start for three forward facing mics is a pair of L/R cardioids or supercards spaced about 2' apart and angled +/-45 degrees and the center microphone facing directly ahead at 0 degrees.
Or if you like spaced omnis, try spacing them up to twice as wide as you normally would (say 4-6 feet) and placing a third mic in the center, which can be any pattern. This also works really well with mono SBD into the 3rd channel.4) A backwards facing center mic can make sense when you are in a good sounding room, are FOB in the middle of the audience or outdoors [edit- also useful when running an L/R pair in PAS without much spacing between mics]. In that case you needn't space or angle the L/R pair more than you normally would, but can add a little more spacing if you want to. This adds a visceral sense of depth, room dimension, and audience involvement.
5) Mixing it: In all these scenarios, first level balance the Left and Right mic channels on their own. Then bring up the 3rd channel, panned to center (or duplicated to two linked channels panned hard Left/Right) and adjust its level by ear. You gain lots of powerful mixing options with this- You can adjust for off-centeredness which cannot be corrected by Left/Right balance alone by panning the 3rd channel somewhat off center. You can EQ the center differently from Left/Right (this is very powerful). You may find that you can make a mix that works a few ways and choose between them: all three mics at about the same level, L/R with much higher level + just enough center channel, or mostly center channel with just enough L/R to achieve sufficient stereo width. With a rear-facing 3rd channel I find I often use just a touch such that its barely heard, but when that channel is muted you immediately miss it as things go flat.
6) Have fun with it. It's perfectly fine if you conclude you are happiest using just two microphones!