Well, I'm not really basing this on any particular body of theory, other than the well-known fact that any solid object disturbs the sound field in which it is placed, at any frequencies above the point where any dimension of the object is equal to half a wavelength of the sound. Lower-frequency waves simply flow around such an object, but higher-frequency waves are subject to reflection and/or diffraction. This can alter both the directional pattern (at certain frequencies) and (at certain pickup angles) the frequency response--characteristics which of course depend on one another.
As you may know, Schoeps sells a miniature stereo microphone called the CMXY 4V which has two MK 4V capsules mounted side-by-side on a gear-tooth arrangement that lets the user set any desired angle between their axes. This can be seen on
http://www.schoeps.de/en/products/cmxy . This microphone works quite well even though there obviously has to be some deformation of response simply because each capsule has a solid object (i.e. the other capsule) sitting so close beside it. Anyway, without going into too much detail, let me just say that a few years ago I had the use of a CMXY that had been specially built with supercardioid MK 41V capsules--and it didn't live up to my hopes, nor those of people at Schoeps; it didn't give a particularly good account of how those capsules are capable of sounding when they're used more appropriately.
So I really wouldn't recommend placing a pair of those capsules directly side-by-side; I think it would be better to place them tip-to-tip, or else to put an inch or two or three of space between them, with appropriate adjustments of the angle between their main axes to match the spacing. Nor would I recommend placing two figure-8 capsules or microphones immediately side-by-side either; head-to-head is a much better arrangement for them as well, since it gets them each out of the other one's way for the most part.
And to the extent that a head-to-head pair of directional capsules still creates some obstruction for the highest-frequency sound waves, at least it is a symmetrical arrangement, so that the directional patterns of the two capsules can still be as intended in the horizontal plane at least.
--best regards