I have several 4 channel recordings that are just 3 channels, although usually it's the SBD and not the AUD that's the problem (some SBDs I patch out of only give a mono feed).
What is "wrong" with the one mic in your mix? That seems like a placement issue if nothing else. Have you tried not using XY and maybe doing ORTF if you're onstage?
Another alternative I like, if you have this issue, is you can make two mono mixdowns but balance them differently, so that the left channel is, say, 75% left channel, 25% right channel and the right channel is 25% left channel, 75% right channel. This gives you the feel of a stereo mix but lessens the discordance between the two channels, since they are blended together a bit.