It's a nice idea, but I don't think it would work too well. I haven't seen the machines that make strings, but as I understand it they are very expensive, and the work they do didn't impress me in the least.
There's quite a bit of twisting to a flemish string, but there's not that much that the device shown could do. I figure you'd loose a lot more time than you gained with it, because you would constantly be swapping it around.
You have a few twists to make the base for the loop--take it off. Marry the tag ends to the main bundles, twist those up--take it off. Back-twist the two main bundles--take it off. Add reverse twist--take it off. Make the base for the second loop--take it off. Marry those tag ends to the main bundles--take it off. Twist up the "body" of the string.
Shoot--you can just about make the string in the time it takes to type it out without the device. Might be fun to play with all the same though.
Chad