I've heard the rule of thumb is about a fist's width apart so you don't abuse your thumbs when oaring, rowing or whatever you do merrily down the stream. I've rowed with as much as about 10" between the handles, and with just a few inches. but prefer a little more than fist-width.
If you're a newbie rafter, try picking up a copy of "The Complete Whitewater Rafter" by Jeff Bennett. This and many other topics are discussed. You can get it off Amazon or from your local raft shop for $15 or so.
Nothing in the world is more yielding and gentle than water. Yet it has no equal for conquering the resistant and tough. The flexible can overcome the unbending; the soft can overcome the hard. - Lao Tse