My experience is that the double string trimmers just use up twice the string and don't do any better job with average yard trimming.
I used a single-string B&D Grass Hog for 15 years until the motor gave out. Bought 3 different 2-string trimmers and returned them.
Anybody still make the 1-string versions? Only 2-string models are sold at Home Depot or Lowes around here.