Hey Misty,
I have a Janome machine and I use my double needles for pintucking and sewing hems on knits and jeans and sewing ribbon on clothing. You have to determine the width you need between the needles when using it for the ribbon.
You can do a faux cover stitch for hems, but it takes quite a bit of practice and you are kind of sewing blind because the right side of the garment has to be facing the feed dog. I've done it a couple of times but not often enought to feel comfortable with it.
Yes, you use two spools of thread and your machine should have a place for another spindle to fit. Thread it just like you would if it was one needle. Surfi and Ticki is right that it doesn't use the zigzag stitch. At least mine doesn't and that would be interesting to see!
Hope that helps a little more.
Susie