The traditional way of making a multi-start screw is to use a dividing head and rotate the part an appropriate angle. If you're working with change gears, choosing a gear that's a multiple of the number of starts could provide alternate way to do this.
Oh yes, flipping the screw blank end-over-end would be an awkward way to do it unless you wanted to make a screw with two starts each having opposite errors. Once the screws have gotten good enough you wouldn't need to bother with things like that.
