I don't think you're really answering the OP's question. I totally agree with you that doing one first (either one) will make it easier to learn the other afterwards. But the OP seems to be asking about learning both or doing both at the same time, and whether that will help either one.
I don't think it will personally. Time spent skiing is time not spent snowboarding. Yes, time spent skiing is still time spent learning how to handle lifts, edges, types of snow, etc, but you won't learn the "common" stuff any faster than you would on a snowboard.
I do think it's a good idea to know how to do both, but there's no speed-of-learning advantage.
Great answer! And sorry about posting this on the Music page...I have no clue how I did that. Anyway to fix that?