We plant 3 feet apart. Farther apart is probably better, but you could get closer if they are the same variety. If they are different varieties, its not "cross pollinating" that you are worried about like David said. Rather you are worried that the bines are going to intertwine and you won't know what variety you are harvesting.
And trust me, saying, "Oh, I'll be out there everyday separating them." won't work. During the height of growing season if you miss just a day of untangling them, you will have your work cut out. They will jump to the other plant and start twisting around each other in no time.
From the rhizome standpoint, put a barrier in the ground to keep the rhizome from shooting out. I've heard of people using plastic barrels without the bottom, metal street signs, boards, etc.