What does double threading do? I have a pattern knit slippers I want to make. I wanted to make them extra warm. Can I just double thread? What will that do? Would I need to use different needles?
What does double threading do?
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!
Double stranding will make warmer slippers, but if you just do it to any pattern you’ll probably not get what you expect. You might be able to knit a smaller size and do fine, but it’ll be a little guesswork. There are some patterns that call for bulky or double stranded yarn and those would probably be best to work with. What kind of slipper are you wanting to make?
You can double strand but if the yarn is bulky then you will have the huge slippers. But if the yarn you choose is thinner than the pattern calls for and you choose to double strand be sure to knit a swatch.