now a lot of tapers have split omnis in the past as far as 15',20', hell....even 30'. however you must take into account of all the phase issues you are going to have the wider they are apart. especially with giant outdoor PA stacks and being that far away. with this in mind i would never split them more than 6' unless i had no other choice due to how the section is set up.
now i think the closer you get to the sound source, ie. waaayyy FOB you might be able to get away with a wider spread depending upon where you are at and how far the PA stacks are apart from each other.