There are basically three variations for recording effective backing vocals, but keep in mind nothing is set in stone. Much like the Pirate’s Code, these are more guidelines than actual hard and fast rules.
Variation 2: When simultaneously recording several voices with a separate microphone for each vocalist, use hypercardioid mics in order to prevent leakage, particularly if the microphones are spaced closely.
Variation 3: When using a single microphone for several vocalists, set its polar pattern to omni or cardioid and place the choir in a wide semicircle around the microphone. On large mixed choirs, use a spot microphone for each voice. You may also consider using an additional stereo microphone. In acoustically perfect environments, a single stereo microphone or a matched pair of mono microphones (ideally, two AKG C 414 B-XL II) will often be enough to make a very good recording.