Great link. Now go to that link and scoll down to the grid. Resize the window. See how all the columns, regardless of span number eventually become 100% of the column width. My example of two span6s is just that, an example. At the phone viewport all columns become 100% of the width. Your statement that "a span6 is 50% of 12" contradicts exactly what happens if you resize the window. And the way the columns behave by following your link, is the way bootstrap behaves, but not the way Pagelines behaves if you use my code.
My point is simple: The code I stated above gives me two columns at 50% width throughout all viewports, as JamesB's image shows. However, it shouldn't be that way. I've developed many things with bootstrap, and two span6s should become, each, 100% of the column at the phone viewport. But with a clean install of Pagelines it doesn't. Why not?