Some general starting points-
Search the term "omni" in the "Microphones and Setup" section of this site and you'll find loads of info.
Optimal omni spacing is very much a personal preference thing, but is also strongly dependent on what you are recording (the type of music, type of ensemble, if there is PA reinforcement or not and the nature of that reinforcement), the environment in which you are recording, and the recording position within that environment. You'll get wildly varying suggestions on what spacing is appropriate, more so than any other microphone technique, even from the 'by the math' type resources.
Generally, the farther away the recording position is from the source, the wider you'll want to space the microphones.. until you get very close, like on-stage or up against the stage-lip, where it often works to go wider again (for different reasons), or narrower (for different reasons).
If you are planning on simply comparing the two resulting recording to decide which you prefer- one made with omnis and the other made with cardioids, use a narrower spacing for the omnis such as other posters here have suggested above. If you plan to play around with mixing them together, it's better to use a much wider spacing on the omnis, say double whatever spacing you find you like best when using the omnis alone or more. The other pair takes care of any potential 'hole in the middle' making that problem non-existant, replaced instead by potential 'conflilcts in the middle' when you have 4 microphones in close proximity mixed together.
I start 3' wide and adjust narrower or wider from there for a pair of omnis alone. I start at about 5 or 6' wide if I have a third microphone or some other array in the center, with the intention of using them in combination. But that's just me.
Americans typically go much wider than Europeans.
Not just in the waistline.
>You can more safely 'go wider' when there is either only one sound source in the center, or multiple sources with plenty of sound from the center.
>You can more safely 'go really wide' from farther back when there is a big PA, since the PA will be mostly mono, countering tenancy towards 'hole in the middle' (even though the PA speakers may be widely spaced, the apparent sound source is strongly centered. By "sound source" I mean "where the sound seems like it is actually coming from" not where you see the instrument, musician or speaker that is actually producing it. Close your eyes and listen to determine actual sound source directions.)
>Safer to 'go narrower' when there is no sound source in the middle (say no PA and a 2 or 4 piece act with the mics at stage-lip, no sources coming from center) and no other microphone(s) in the middle.
There are other omni techniques besides A-B spaced pair:
>You can place a baffle between two omnis, which allows (and requires) you to put them much closer together, potentially even right up against the baffle. The baffle technique typically won't work if the mics are more than a couple feet apart since the baffle is then too small to do anything. Search "Jecklin-disk", "J-disk" and "baffled omnis" for info.
>Sometimes that baffle can be your own head. Search "HRTF"
>You can "boundary mount" them directly against a hard flat surface which provides unique characteristics (A-B spaced on the back wall, or spaced on the stage floor) or onto several smaller surfaces with an angle between them, making them hemispherically directional from each surface in the higher frequencies (say onto two adjacent sides of a wooden crate or taped to an unused stage monitor) An old PZM trick was fixing the mics to rigid plastic sheets arranged in a 'V' shape producing an omni technique somewhat more like near-spaced directional microphone configurations.
>You can use more than two omnis, three four or five arranged in a line, three in a triangle, or four in a diamond shape. Search "Decca tree".
Have fun!