But website design can be daunting. We want it to be pretty, catchy, and at the same time functional. And, especially with blog sites, there is simply too much we want to get across that doesn't change from post to post. I do get the understanding behind adding a second sidebar. It adds more space, even ad space, to the site (though I imagine having ads in the second bar isn't as bad an issue).
But it also frustrates readers. Readers are only willing to read so far into your sidebar before they get bored or their eyes glaze over. There is such a thing as too much.
Basically, what I'm saying is that if you have so much on your sidebar that you're tempted to add a second, maybe you should be asking yourself what you can do without?