A Double Yarn Over creates two new stitches and results in a much larger hole than a single yarn over. This video demonstrates a Double Yarn Over between knit and purl stitches for both Western and Continental knitters.
Double Yarn Over (yo twice)
A double yarn over will create a larger lacy hole. The make one stitches will not make a hole and the design will be dramatically different. Hope this helps!
Can I substitute m1r,m1l instead of a double yo and would I do the m1r first or the m1l first