Here are a couple videos - include one in slow motion - of Chad Hedrick doing the double push on inline skates. Think you can do this on skis? According to Thomas Stöggl & Stefan Lindinger, the double push on skis is faster than the traditional skating, albeit at a higher energy cost.
Chad Hedrick, Double Pushing in a Race:
Chad Hedrick - Slow Motion double pushing in an inline race:
(Still want to learn more? Here's 9 steps to learn the double push for beginners from the Nett Racing web site).