There are a couple of ways you can go about making really thick, warm slippers, but double strands may or may not be what you really want. If you use the same pattern and needles for two strands of yarn that you use for a single strand, you will get HUGE slippers. Smaller needles would give you a very dense fabric. I'm still not sure you'll be able to get it down to single-strand gauge, but if you have the needles handy it's worth a try on a few stitches first.
The best way to see if two strands will give you what you have in mind is to try it on a swatch, see how many rows and stitches make up four inches, and compare that to any pattern you might be using. You might have to knit the directions for a smaller size, or you might be able to find a double-strand pattern like the one you have. Bulky yarn is a lot of fun to use, too, and there are some nice, inexpensive types out if you can take time to go pet some yarn Knitting Pattern Central has TONS of free slipper patterns. One of them will probably be just what you want!