Hm, I've never tried using two balls... I'm intrigued.
Another way to sort of gage how much tail you'll need is to cast on 10 stitches, mark where your yarn usage stops (if you don't have too much of your tester tail left, just trim it close to the needle for easy marking), then undo and measure how much was actually used. Then you'll know roughly how much tail you'll need for ten stitches, times how many stitches you need, etc...
So: 10 stitchs needed 3 yards of tail from the slip knot (I use a slip knot, some people don't), and I need 300 stitches, so 300 total stitches / 10 test stitches = 30 repeats of 10, so 30 repeats of 3 yards = 30 * 3 = 90 yards of tail.
Add a little extra for good measure and you'll know about how much you'll need.