My question is what is the best way to get full advantage of this machine, to pull both sides at the same time or to pull one side at a time?[/quote]
Eee-ther, eye-ther. Which is better, shoes with laces or sandals? They’re two related options that work towards the same goal, with minor differences.
That’s perfectly normal, but might not be what you want. As weight increases beyond a certain point (whatever you can handle for the full range of motion), the ROM will naturally decrease. It’s the same reason that the stereotypical quarter-squatter in the gym thinks he’s hella-strong while using 315 for a four-inch range of motion.
If you’re doing it consciously and intentionally, that’s fine. But if you don’t realize that your sacrificing ROM for load, you’re fooling yourself. (That’s the general “you”, not “you = Ronda”.)
If you don’t have any other unilateral back exercises in your program, I’d say go for the 1-arm version. You’ll only be “cheating yourself” if you twist so much that it reduces stress on the back. You’ll probably notice this by decreased strength progress and/or possibly less soreness in the back (and maybe more soreness in the abs or shoulder).
And anyhow… why are you doing them? What’s your training goal? If you’re training primarily for strength, a little more body english is fine. If you’re training for hypertrophy, I’d keep it more strict.