It's been rumored for some time that Microsoft will be separating Cortana and search on the Windows 10 desktop. It would seem that it's finally happening. As spotted by Michael Reinders, it's actually showing up in the latest Windows 10 Insider Preview, which is build 18298.
The feature seems to be in A/B testing right now, so only some Insiders are seeing it. Having separate buttons for Cortana and search would change the way users do both, and that's the plan. Ultimately, Cortana is meant to be an assistant across Microsoft's services, rather than a standalone digital assistant. We're already starting to see it integrated into various products across Office 365 and more.
Presumably, this change will make it into Windows 10 19H1, which is due out in the spring. It's still A/B testing though, which means that anything can change. We've seen this with Sets before; sometimes experimental features show up, and then they disappear in favor of the next development cycle.
Source: Michael Reinders (Twitter)