This is a good basic tutorial for this. I learned how to do this years ago for my 3rd project. You can make reversible Fair Isle patterns that look amazing on both sides.
Double Knitting - How to Knit Two Layers of Reversible Fabric at the Same Time
Scenes of Winter reversible double knit blanket pattern by Gertraud L. Hurley
Ravelry: Rainy Day Umbrella Scarf pattern by Christina Drury
Double Knitting. Two approaches for casting on and binding off. ..also tutorials on basics, shaping and more
Reversible Double Knit Rose Scarf FREE PATTERN generously provided by Judy
Sunita's New Yorker For Sunita in New York Double Knitting made with Noro Silk Garden Lite 4 balls ...
Laura's Loop: Arctic Wrap - The Purl Bee - Knitting Crochet Sewing Embroidery Crafts Patterns and Ideas!
Double Knitting Reversible Herringbone Scarf - free pattern
Nordic Scarf - absolutely beautiful colourwork - knit in the round so double-thickness.