A solution may be to place an extra pair of cards on the same bar.
Should this extra pair be a standard DIN or ORTF configuration or should this extra pair match the SRA of the omni pair?
The real-world answer will be to try a few different things, listen carefully and decide how well it works or not.
The SRA (Stereo Recording Angle) for an omni pair at 1 meter distance is 60 degrees (as per the Neumann app "recording angle calculator"). The SRA for an ORTF pair is 96 degrees. Should the SRA's of two pairs on the same bar match eachother?
The part I've bolded above is a question I think about frequently. I've found no clear answer to this yet. As far as I can tell the answer is..
maybe.. but not necessarily.. sometimes best if not. Here's one thing: Even if SRA of two pairs is the same, the image distribution
within the SRAs of the two different configurations is going to be somewhat different. How much does this matter? Probably depends on the situation and content. With an organ recording where image is generally diffuse, probably not as much as other things where sharp imaging is expected.
Here's another thing more fundamental altogether: Mixing the two pairs together will produce a new SRA that is going to be different than that of either pair in isolation.
One take away is that when introducing an additional pair in the middle, it can be a good idea to try spacing the omnis more widely. The best solution may end up being anywhere from the same 1 meter width you are currently using up to twice as wide at 2m. Using the Schoeps Image Assistant visualizer you can see this for yourself by switching between a two microphone configuration and a three microphone configuration that both use the same spacing between the wide pair, noting how the introduction of a third microphone in the center effects the SRA.
Here are a few guidelines from my experience: If you don't have the ability to monitor and adjust the setup prior to recording, you can reduce the chances of bad phasiness occurring by using a coincident pair arrangement in the middle between the omnis instead of a near-spaced pair. That reduces the phase interactions to three different microphone positions in space rather than four. Sorry for being repetitive about this, but its a fundamental issue that becomes important when one cannot monitor and readjust things prior to making the recording. A near-spaced pair can certainly work in the middle, its just more likely to have issues when mixing the two pairs together which you won't be able to discover until after you mix them. Its also less likely to be problematic because it is combining two stereo pairs that behave sufficiently differently from each other - one producing stereo cues based strongly on time of arrival differences, the other solely on level differences - making them less likely to "step on each others toes".
Somewhat OT but related-
I'm currently working on a 3-position version of the 2-channel stereo Improved PAS table I posted at TS years ago. The idea of the original Improved PAS table was to create a list of stereo microphone pair spacing/angle combinations for various Orchestra Angles (OA - which for PA amplified things is the PA angle) where SRA is always equal to OA (more precisely, OA + 10 degrees). This simplifies the optimization of a microphone configuration from a restricted recording location which may be more distant than desirable by pointing the microphones directly at the outer edges of the source, ensemble or PA and adjusting the spacing between the microphone pair based upon whatever the angle between them ends up being. The new 3-position Improved PAS table extends this to microphone arrays of 3 or 4 channels, 3 if using a single microphone in the center, 4 if using a coincident pair in the center.
^ A couple general takeaways I've noticed while working this up are:
The spacing between the wide pair is always larger (up to twice as wide as mentioned).
The center single microphone or coincident-pair is typically best placed about 20cm forward of the wide pair.
If you end up trying a few different things, please let me know what ends up working best for you.