there are dual-layer ones, see here http://crediar.no-ip.com/rwtr.jpg see it saying "DISC(SL)" and other times it says "DISC(DL)" obviously means single and dual layer.
I think that dual layers exist for sure. The devs have to present theirs latest builds on physical media no? so i'm pretty sure that games like Samurai Warriors or SSBB were burned on dual layer discs, even if it was just for some tests. "Single layer" words are written on all common RVT-R discs. It must be because dual layers are produced too. Now 1 retail game on 100 is manufactured on a dual layer disc, so theirs RVT-R version are rare too..