Has anyone seen a tutorial on the technique used to ice a stacked wedding cake with chocolate buttercream on one side and vanilla buttercream on the other?

I saw something once, I believe. Really simple. Iced the white side, more than half, and even 3/4 way around the cake. Cover the "white" half with waxed paper to protect, and ice on the dark side.The waxed paper protected the "line" and kept them separate. I do believe some "border" was added to visually seperate the sides more as well.

Thanks that makes sense.  Do you remember if each cake was iced separately or once all the tiers were stacked together.  If separate, I'm wondering how to measure correctly so the lines from each cake line up properly--or is that why you ice 3/4 of it with white to give you some buffer for alignment.

Iced when stacked. Exactly... the "overlap" is a "buffer" zone, meant to be overlapped. You just have to be exact when you cover with parchment or waxed paper to get it 50/50.