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.
Totally. As a person who learned to ski first the only real advantage it gave me was that I was comfortable sliding around on snow. The two are pretty distinctly different since 4 edges is certainly not the same as 2. I'll say this, my first pow day on a snowboard made me a permanent convert.