Primary display when using dual-head setup

What is the mechanism by which unity determines what the primary screen is if I enable an external monitor? When using the nvidia-settings utility to enable the external monitor in TwinView mode to be "left-of" the built-in monitor, I always have to do so twice in order for the launcher and menu-bar to appear on the external monitor (i.e. the procedure is 1) enable external monitor 2) disable 3) enable external monitor). It is consistent in that I have to always do it twice, but also a bit disturbing. Also, the "Make this the primary display for the X screen" tick doesn't seem to have any effect...

Is this intended behaviour (if so, is there a way to tell unity to pick the external monitor first) or a bug?