Hi Hill,
You've got it right-you will be working with two strands held together, knitting with them as if they were a single strand. Make sure you cast on with the double strand, and try knitting a few rows before you begin the pattern.
It takes a little getting used to, but you should catch on quickly. It does require a bit more concentration, however, to ensure that you don't drop one of the strands.
Using two strands is alot of fun, by the way, and can be a fun way to create easy and interesting effects with your yarn. It can also be a great to substitute or use up yarns. A good rule of thumb for estimating what gauge two strands will work out to is to multiply the suggested guage of one strand by .72.
For example, a single strand of a sportweight yarn might be 24 sts/4 inches. If you were to use two strands of this yarn, your gauge would work out to roughly 24 x .72 or about 17 sts/4 inches. That means it would probably work with a pattern calling for aran weight (17-18 sts/4 inches) yarn. As always, be sure to do a gauge swatch just to be sure.
Best of luck with project, and we're glad to have you here!
Happy knitting,
Alison