Band and choir at the same time? What a headache. I have my annual gig doing simultaneous FOH/recording of a 150-voice women's chorus and a 7-pc Dixieland band coming up Dec. 1, and I get hives just thinking about it.
What I do is try and get the chorus up in the FOH mix to balance the levels with the band, and then do a good XY recording of the room from about row 10 in the audience, live to 2 track. I separate the FOH and recording chores _completely_. This may not work for you, but it's the only thing that works for me: trying to do both sophisticated live recording and FOH with a paying audience in the room is too high a workload. If I can get it to sound right in the room, then the paying customers get their money's worth, and the live-to-2 recording is then pretty much a nobrainer. Trim the levels, stick a stereo limiter on the feed to handle the applause intervals, and then forget it and mix for the house. The performance is more important than my recording of it, in that setting.
I mic the chorus with 4 or 6 hypercardioids hung from overhead and pan them in the FOH mix for the right separation (yes, I do do a stereo FOH mix for this occasion). I need the hypers because the band sets up right in the big-ass middle of the chorus risers: trying to do anything with a loose pattern just leads to bleed and mud. I use Oktava MC012s for that chore, and AKG C535s on stands for any vocal soloists. The miking I use for the band doesn't apply at all for your situation, unless you have a tuba to deal with- I finally found the perfect tuba mic... (;-)
Are you doing this as part of a live gig, or do you have the luxury of mucking up the sightlines in a more studio-like setting- and even, perhaps, setting up in the round? If the band is electrified, you may have the same problems I do getting any kind of a balance with an XY setup: the band just roars over the top of all the vocal subtleties. You may need to try splitting into sections and micing with a tighter pattern (although perhaps not as tight as the hypers I have to use). If it's not live, you can try goboing the hell out of the band as well. A simple XY setup in front of the choir is likely to just get _killed_ with the bleed from the band. But, just the same, the chorus is probably used to singing against that very roar, and will have a hard time with tuning and phrasing if they _don't_ get that. You have to work with what they need in order to perform, first and foremost, and then figure out how to print it.
Definitely set up the XY pair and print it to 2- that's your safety. I just doubt that it'll be that useful, unless you can set up the band directly behind it (in the round: mics in the middle, performers all facing each other). and back up the pair with a gobo...
And I'll bet you a beer that the choir would sound like hell if they were trying to monitor through headphones. Large vocal ensembles are _extremely_ sensitive to the acoustics of the space they are performing in: take that sense of space away by clamping on cans, and you get hammered dogmeat on wheels- not music!
This combination makes your job as recordist very, very hard. Sometimes, you just have to get them what they need, and then work with it, bleed and all. Keep a selection of tight mics available, is all I can say. And use your ears in the room; creative placement will help a ton with the inevitable bleed issues.
Hope that helps a little: every situation is different, just like every room and every group of performers...
