While you can do this, at the moment Opus doesn't provide a way to keep a toolbar aligned with a file display, so if you resize the window it'll look quite ugly.
If you still want to do it (e.g. you don't care if it looks ugly, or you always used a fixed-size window layout) then shout and I'll explain or dig up an old post (I think there is one).
An alternative which you might find better, though, is having a single drives toolbar which you can left or right click to open drives in the left or right side. There are a couple of ways to do this but the easiest is using the Go DRIVEBUTTONS=multifunc command.
If you haven't already, check out the video tutorial on Toolbar Editing to see how to use the above command.