How I did it:
1) Paint the entire nail with the dark color (Sapphire Siren). You may need 2-3 coats. Let dry.
2) Once dry, paint a stripe down the center of each nail with the lighter color (Shocking Seas). Again, you may need a couple coats. Let dry.
I love accenting a deep blue with a lighter blue, and these shades are bold but not overpowering.