I don't think I've ever seen DDG show up in my logs, and I found this article from a web host that listed out their top crawler traffic and DDG doesn't even register (but something called Sogou is): https://deviceatlas.com/blog/most-active-bots-and-crawlers-w...

It might be that DDG sticks to the to X thousand sites or something like that, so I would imagine that Bing must be providing the majority of the index, but maybe not for more popular queries.