Well, it's probably prudent to swatch whether you do the math or not. It'll give you good starting point, though.
Kellyjo, if you do decide to double the yarn, you can multiply the original yardage requirements by 1.4 to get the new yardage. E.g., if original pattern required 100 yards, you'd need 140 yards of the yarn you plan to double. This is assuming you are maintaining the same gauge, of course.
Oh yeah, don't use these formulas for novelty yarns because they probably won't work. Forgot to mention that the first time...
Another little tidbit... as a general rule:
Two strands fingering equals one strand sport.
Two strands sport equals one strand worsted.
Two strands worsted equals one strand bulky.