I tried to use a 2 uni directional capsule facing away from each other in a stereo mic(to be used for my MP3). The result is I got a response of a noise canceling microphone. Is this usual the outcome for this kind of capsule combination? What about other capsule combinations (omni, bidirectional)? Please help me come out with the right answer.
The Shure VP88 is a one piece stereo microphone which will solve the problems you encountered. You can also use two Shure unidirectional mics, like the KSM137, and mount them on a Shure A27M adapter.
If you wish to learn more about creating your own stereo mic pairs, consult the book: Stereo Microphone Techniques, authored by Bruce Bartlett; ISBN 0-240-80076-1 This is the most complete book ever written on stereo miking.