You have to train separate shoots, cutting the tip can easily kill the shoot. If this is your first year, you may only see one per rhizome. Yield depends a lot on the root system.
actually I was hoping to do it when the bines were about 6' tall later this year. I am no botanist but it would seem that if a bine is hearty enough to be that tall snipping the tip would force the bine to split. In my mind it would have enough transpiration going on and root development at that point to just power through and split.