I would say that the very minimum would be 18 inches apart without a baffle
i have run 3 ft. and i have run 30 ft. and various distances in between, depending on how secure the area was.
DPA website says you should space them 3 ft. apart for every foot from the sound source. If course, that isn't very practical, mic cable wise, nor for securing the position of both stands...
anyhow, once you reach 15 or more feet from the source, i think that the ratio you want to travel apart falls off at a pretty steep curve.
if you want to hear 2 different distances apart - 18 ft and 3ft check out these recordings... i ran 3 ft apart on the sunday show because it was raining, and i had a 3ft t-bar, which made it easier in the rain.
http://www.archive.org/audio/etree-details-db.php?id=26827http://www.archive.org/audio/etree-details-db.php?id=30832edit to add: the j-disk is the way to fly if you want to run omnis close or in a small place.