^
Any decent stereo recorder can record Mid and Side (and produce a great Mid/Side recording). There is need for a machine with Mid/Side decoding built in to do so. Far better to determine the most optimal matrix ratio between the two afterwards rather than on the fly. Just be sure to route your Mid mic to the Left channel and the Side mic to the Right channel for proper decoding afterwards.
Its true that the two onboard omnis are too close together for full stereo image.
Thats why I will combine it with a set of cardoid f.e.similar to ORTF. My hope is to get the good responce of the omnis for low frequency range especially for organ sound and the good stereo picture with the external cardoids. That is the reason I asked for accoustical quality of the onboard omnis-
Grosse_j
If you must use the internal omnis, figure some way of rigging up a baffle between them to improve separation. Otherwise, and probably in addition to that, compensate with your external microphone pair setup in such a way as to over-emphasize stereo-width / stereo-difference information (tending toward "hole in the middle"-ness when monitoring that pair in isolation). ORTF represents an
"optimized for 2-channel recording microphone technique", and when mixed with closely spaced internal omnis will produce a rather monophonic result. Assuming the ORTF 110 degree angle between microphones is appropriate to the room and your recording position in it with respect to the source, space the cardioids significantly further apart than ORTF's specified 17cm. I'd space them at least twice as far or more if mixing the internal omnis in full-range.
Alternately, if you plan to low-pass the omnis and chop off their contribution above 100Hz or so prior to mixing with the ORTF cardioid pair, standard ORTF spacing may be appropriate (again, assuming ORTF is the best choice given the room and your recording position in it with respect to the source).
However, I see all of the above as compensation for incorporating overly narrow-spaced omnis without introducing more problems than the benefit you get from using them at all.
The far superior answer will be to pick up a pair of external omnis which you can setup optimally. Even if they are inexpensive and nothing pedigree they will be on par with the quality of the built-in mics yet provide the needed flexibility to do a proper microphone setup with the omnis spaced significantly further apart (at least twice as much as the directional pair you plan to mix with them, preferably more). Mid/Side works very well in combination with spaced omnis. In my experience, ORTF as an
"optimized for 2-channel recording microphone technique" does not work as well mixed with a pair of omnis, and an alternate setup using the cardioids with a narrower angle or less spacing between them is likely to be more appropriate. Note that this represents the inverse of the situation described above when considering mixing ORTF with the internal omnis that have insufficient spacing between them.
Via trial and error you may be able to tweak the omni spacing "just right" to mix nicely with ORTF, but I tend to think of ORTF being best when used on its own, and find alternate stereo configions superior for combination with omnis.