Hello Martian,
When you twist the pairs together, you will lower the resistance, which is always beneficial. You can't harm anything. If you are running 30 feet of cable, I'd use 14-gauge or thicker. But I suspect your twisted 16-gauge pairs would about equal the 14-gauge.
If you really want to get techie, go buy a cheap Radio Shack digital AC/DC volt-ohmmeter and actually measure the resistance. It's fun! So long as you keep the total resistance (both directions) to 0.3 ohm or less, you have no problem. Those little ol' electrons will zip along quite happily at the speed of light, not knowing at all whether they're "bi-wired" or "single-wired"!
Regards,
