I have never co-written, but Oz tells me it's hard. You have to meet on AIM/MSN frequently to exchange ideas or post chappies to each other, you have to AGREE (something I find hard, I'm the type who pulls out the wild cards when it comes to writing^^) and it takes a long time.
Well the Electric Buggy To Victory and Lisa The Legend Crossover's (there have been 2) were a joy to read. Those writers are an asset to this forum.
I was going to write some co-work with Subsane, but he's gone...somewhere...