Two strands of the worsted weight would make a very bulky/chunky yarn, though I'd knit it with 10 mm rather than 9. If you have very thick yarn you could use a single strand of it, or if you have thinner yarn, you can use 3 strands together. Try out different combinations until you get a fabric you like that isn't too densely knit or too thin. You might use 2 strands of a thinner yarn, and smaller needles, like 7 or 8mm and increase the number of sts to make the same size afghan.
sue- knitting heretic