That wouldn't make any sense given that DDG is thin UX veneer around Bing, and it doesn't maintain its own index.
Tsignal [1] claims to be building their own index, which if they are would mean there was a third index to choose from.
[1] https://www.theregister.co.uk/2017/08/31/deepsearch_tsignal/
Another area where table stakes are incredibly high is geo (maps).
What’s going to kill you is ranking (especially in the tail) and keeping the index fresh, including the tail. Ranking requires human input. Keeping it fresh requires some non-trivial effort in the crawler too. And then there’s also serving all of it at 1M QPS, as a prerequisite for creating enough ad revenue to sustain the effort. That requires predictive caching. As a result you get a couple thousand people working on the thing, each pulling down about $400k on average when you total everything up. That’s $800M right off the bat, unless you cut corners like Bing (cleverly) did by stealing your competitors clicks as weak relevance supervision.