The DC Plus can pull two strings at one without slipping at the tensioning head. Just make sure both strings are in there. Once that's done, clamp one off and pull tension again. If for some reason, the string slips at the clamp, then you're not clamping down hard enough on the string. Tighten the clamps a bit more and try again. Do it just a bit at a time until you find the best hold while not damaging the string. It takes a little testing around, but eventually you'll find that strings are more durable than you think.
BTW, strings tend to slip when you go crazy on the weight. If you're stringing 60# worth of weight, then that's probably your problem. Either string lower or just tighten your clamps. I find that a lot of people string a lot higher than they actually need to and don't even know it.