ScottT, when people say "X/Y" with no further details, they generally mean a coincident pair of cardioids. Coincident stereo recordings made with cardioids tend to have high positive correlation between the channels, since for one thing a cardioid is quite a broad pattern, and especially since there is this nutso idea out there that 90 degrees is some kind of ideal angle to set between the axes of a pair of X/Y cardioids (try 120 instead and be amazed).
A high positive correlation between channels is a Good Thing for mono compatibility, but it detracts from any sense of spaciousness in the recording. I generally don't recommend using cardioids in a coincident setup unless mono compatibility is an absolute requirement; I generally prefer ORTF for cardioids, and I only use coincident placement for M/S or for patterns such as supercardioid or figure-8, which are narrow enough to create clear differentiation between the stereo channels.
Placing two cardioids together doesn't decrease their low-frequency sensitivity as such, but it does lump much of the good stuff in a stereo recording (both direct and reflected sound energy) dead into the center of the stereo image, giving a "flat," basically mono feel to the recording.
--best regards