But some TV shows (Burn Notice, for one) do this frequently. It works pretty well.
Try going to Fry's and finding a spot where they have 2 TVs next to each other that are showing different channels. Try just watching one of them: It's maddening. To me it's equivalent to animated ads on web pages--it just makes it impossible to concentrate.
However, the thing I find interesting is other people in the comments here have mentioned that they just look at the center and take in what's going on on both sides, which is exactly the same technique, and Facebook timelines are sparse in the same way that video is. It's easy to get a sense of what your Facebook friends are doing with a quick skim, without fully concentrating on any particular post. During this skim, your attention might be drawn to a specific post, but if it's not, you've absorbed a high-level overview and moved on, without having to read each post individually.
There's not really ever 2 concurrent plots happening simultaneously.
1) Lines are used to divide content and guide our eyes to what is important. In this case, we have to travel "across the line" to see the next piece of content which may be related, which breaks common notions.
2) Our mind usually focuses on the largest items on a page and then goes to the smaller details afterwards (Like how magazine article titles are always bigger than the text below it). In this case, after the top photo, my eyes move towards the bigger content boxes than the thin line in the middle.
EDIT: A third one!
3) If we focus on the line, the content to the left of the line becomes slower to read than the content to the right of the line. When we look to the left of the line, our eyes have to travel across the entire box to get to the start of the status. When our eyes travel to the right, the status starts a lot closer (due to text being written from left to right)
Here's an example:
Its a great day to be a programmer | I can't believe I got this job!
If you read this left to right, its fairly to read. But if you start in the middle at the line and want to read the left comment, your eyes have to go all the way to the left to get to the beginning, then come all the way back to where they were in the middle at the line. When you read the comment to the right, the text starts immediately.
Newspapers work well with multiple columns because there's no inherent connecting features. Sure a page from the sports section will all have sports related items, but in general all the columns stand by themselves.
Facebook timeline is united by time. As in, what I say at the top of my timeline might be directly related to what I said before, and that might be directly related to what i said before that, and so on. So as humans, we want to read objects in the way they were chronologically presented. But on the new timeline, in order to do this, we have to zig zag from left to right, to left again. This isn't engaging the user just as making me do jumping jacks while I read isn't engaging -- yes it is spicing up my reading experience, but that just forces me to relearn something that is harder than what I was doing originally.
If the timeline was broken up into pieces, I could deal with that. Wall posts on the left, images and polls on the right, etc. Give my eyes some logical way to focus on only a part of my screen. But the exact middle of the screen on Facebook is a dividing line between left and right that presents almost identical information, and I have to form a structure from that.
Taking another example, in movies, it makes sense that the movie makers try to confuse and sex up a movie -- our brains want to be stimulated. But there's a difference between spicing the content of entertainment and spicing up the delivery method that content is offered -- imagine going to the movie and having two videos on the screen at once -- both showing different things. I'm pretty sure you wouldn't be happy.