With professional stereo condenser microphones it is generally necessary to power both channels. If you power only one channel, generally only that one channel will work--or in a few cases the entire microphone will operate poorly or not at all. With professional stereo microphones it's more common for the two channels to have separate amplifiers and powering, so that the one unit can be used as a dual-mono microphone ("a backup for itself").
With semi-pro and consumer microphones there is more variation (standards are respected less, or in some cases don't exist), but I've never heard of any stereo condenser microphone that should not be powered in both channels, i.e. for which you'd need specifically to avoid powering both channels.
--best regards