To make a stereo micing like the ORTF you need to identical condensers preferably matched pair, is not a good idea to have different mics in it because there will be differences in polar and frecuency response that will produce a bad stereo image (example: one side will be more bright than the other, or youll have a blury stereo image, things like that) an ORTF is a stereo technique implemented by French Broadcast engineers a long time ago, this is achieved using two condensers, making an angle of 110 degrees (or 55degrees each one) betwee both mic capsules and 17cm space between the capsules. So the result will be two mics separated 17cm (the capsules) one pointing left in an angle of 55 degrees and the other pointing right in an angle of 55 degrees, left mic goes to one channel panned to the left, and the right one to another channel panned to the right. Experiment with mic positioning (closer or far to the ensemble) and height (looking from the top down to the ensemble or about the musicians heads pointing towards their instruments, and stuff like that)