I've had good results with a 4 to 5 foot split.
this was jazz/funk/improv/space group (for lack of a better term), maybe 5 or 6 players on a fairly small stage.
the key to running on-stage, regardless of cards/omnis/sub/cards/whatever polar pattern you might choose, is to be very aware of where the mics are in relation to drums, amps, etc, etc. You are going to be much closer to the individual sound sources, so it can be a bit tricky to get a good mix of instruments. (i.e. its very easy to get too much drums, as mentioned above. but you also don't want to setup right next to the guitar amp, for similar reasons).
in situations where you don't have much choice for setup, like if you're going to be closer to the drums than you'd like, I've had good results running cards ORTF. the drums will then be hitting the mics not very on-axis, and will therefore be attenuated a bit, which helps with the overall mix. of course, with omnis, you don't really have that kind of option, so it's even more critical that you get the mic placement right.