Omnis typically work best outside or up close in a good sounding room. Unless you place a baffle between them, they work best when split more than a foot in almost all situations. Using omnis only I'd say space them between 15" to 3' to be safe, but that's just a general guide, sometimes wider can work. When combined with a center X/Y pair, the omnis are best splilt farther. Say 3'-6'. Depends on the situation though.