Probably means that no one knows a way - that does happen sometimes.
I don't know a lot of HTML/CSS, but I did spend a little time looking in to this once and didn't find any magic "put this in two columns" feature. I think the only way to do it using the threads in the order they're generated would be to make each column a table column, but if you did that then the cells probably wouldn't line up side-to-side, and it wouldn't form a grid. So unless I missed something (which is possible), I'd say the only way to do it would be to do some significant modification to showthread.php.