The Nexus 6P. Photo: Google.

Since 2010, each new version of Android has brought with it a new Nexus smartphone.

The latest version of the operating system, Android 6.0 Marshmallow, is a little different. This year, for the first time, Alphabet's Google (NASDAQ:GOOG) (NASDAQ:GOOGL) is launching two new Nexus phones simultaneously -- the Nexus 5X and the Nexus 6P. Previously, it had launched only one handset at a time.

Nexus phones have never been big sellers, but this shift, along with a new program, could bring them more into the mainstream. And it should help Google's upstart mobile virtual network operator, Project Fi, achieve a greater level success.

Project Fi's handicap
Project Fi has many intriguing aspects for consumers. It's relatively cheap and fairly straightforward -- subscribers pay only for the amount of data they actually use. The same phone number can be tied to multiple devices. Best of all, it supports two networks. Project Fi switches between T-Mobile and Sprint's wireless networks on the fly, choosing whichever has the best signal in any given area. On their own, Sprint and T-Mobile don't offer as much coverage as Verizon or AT&T, but combined, the gap narrows. (NYSE:T)

Yet Project Fi has been held back by a major limitation -- it only worked with a single handset, last year's Nexus 6, which is a truly massive phone. Earlier this year, a Google executive told The Wall Street Journal that, for all practical purposes, it was basically a tablet. Even fans of big phones may find it too large, as it's bigger than both the iPhone 6s Plus and competing Galaxy Note 4. Phablets are increasingly popular in the U.S., but smaller models continue to outsell them.

This year's Nexus phones are more attractive devices, with more broadly appealing physical dimensions. The Nexus 5X sports a 5.2-inch screen, the standard for Android flagships, while the Nexus 6P, at 5.7-inches, is on par with most modern phablets, including the Galaxy Note 5. They're also cheaper. At $649, the Nexus 6 was somewhat expensive when it debuted last fall. The Nexus 5X starts at $379; Nexus 6P at $499. Both support Project Fi.

No carrier support
In fact, Project Fi is the only carrier the Nexus 5X and Nexus 6P will be sold through. If you sign up for a new Project Fi account, you're prompted to order one of the two handsets and pay for them in monthly installments. The phones can also be purchased outright through Google's online storefront, Google Play, and used with any of the major wireless provider, but unlike last year's Nexus 6, they won't be sold in carrier stores.

Google is also offering a new program for these Nexus handsets -- Nexus Protect. Like AppleCare, Nexus Protect gives buyers the ability to extend the warranty of a Nexus phone to two full years and add accident protection. It may seem like a small change, but it offers peace of mind for consumers who may have relied on carrier-provided handset insurance in the past.

Google has downplayed its Project Fi ambitions, saying that it doesn't intend to become a wireless carrier "at scale" or challenge any of the major providers directly. The Nexus 5X and Nexus 6P won't propel Project Fi to tens of millions of subscribers overnight, but they certainly make Google a much more attractive wireless provider.