I wanted to have a stab at it myself and turned to youtube to see what I could find. There are lots of tutorials on gradient/two tone nails but I went along with the techniques as seen in this one.
I used Barry M's "Berry Ice cream" as my base coat and Collection 2000's "Dynasty" and George by Asda's "Streamer" as the two top coat colours. I didn't have a makeup sponge so I used a washing up one that I cut up.
This is how it turned out - splodgey and without a gradient, which is because the sponge I used was coarse. I am going to buy some cheapo makeup sponges (which are much smoother) and will post my second attempt in the near future. Now where's my nail polish remover.......