If you want to get any stereo separation, you'd be better off using a pair of cardioid (directional) mics, or a single point (usually condensor) stereo mic - some of which have a "wide" setting. Most cardioid mics are not that directional and have fairly good pick-up for at least 120 degrees in front of them - they usually have a diagram of their characteristics with them. Nearly all recording devices of any quality (MD, Tape, or PC) will have a stereo input so you will have to make a "Y" connector up if you use a single mic - mind you you will need to make up a lead if you intend to put two mics straight into a single stereo jack socket.

It's strange thinking of omnidirectional mics as expensive - what used to cost the money was the degree of directional response that a mic had.

Most of the boundary microphones which I've seen, are intended primarily for speech and don't have a very wide frequency response for music.

I would recommend using a mixer between the mic(s) and the recording device - this will help you to set a suitable gain (sensitivity) level.