I suspect the limitation is that the SOC doesn't have the IO bandwidth to support two ports at usb 3 speeds (remembering that the SOC was designed for iphones which physically only have one port).

Ah, that's a good point, it would make sense (and be a small but real gotcha of using a phone CPU in a laptop).

Does this mean it is now possible to run OS X on an ipad? Those also tend to use phone processors.