There's is a tut here: http://www.squidoo.com/how-to-make-a-double-layer-tutu
for a simply single layer tutu. Scan down to the part with the heading **How to make a tutu - No sew version** it shows you basic instructions for how to do a slip knot on a tutu.
I can't seem to find the instructions for the double layer but it's simple. Just take your two strips of tulle, one in each color you want. Link them together by folding them in half and linking each piece at the fold. Slip it under the elastic tie it like you were going to tie a shoe pulling the two pieces apart horizontaly, then tie another knot but this time pulling them up and down. So when you've let them go you'll have one color on top and the other on bottom. You can keep them the same length so it's reverasable or you have the top layer shorter soyou wind up with a two layer.
I'm sure that doesn't make much sense but it's hard to explain. I'll keep an eye out for those instructions