Earlier this year, Gary Illyes from Google mentioned that Google has been working on a completely separate mobile index.  This would see both desktop and mobile have its own set of search results which would be served depending on what the searcher was searcher with.

Google is reportedly still testing two indexes as Maile Ohye from Google reveals that there are some issues with Google not always able to show the correct URL.

One of the issues appears to be sites with pagination – ie. a site that spreads out 10 tips across 12 pages, or is just lengthy content that is split into multiple pages.

The one thing that concerns me about the possibility of Google switching to two separate indexes, one for desktop and one for mobile, is that it could potentially favor sites that use the two URL structure for mobile and desktop, since it would be much easier for a webmaster to independently optimize a separate mobile URL for the mobile index, such as tailor snippets and title tags for mobile and for desktop.

However, on the flip side, the two URL structure is often the one with implementation errors, if site owners do not correctly link the two pages together.

This will definitely be something for SEOs to keep an eye on, because there definitely are implications to be considered if there are two different indexes, with what would very likely be different SEO tactics for each.