Between them, Instagram and Hipstamatic have 31 million users. Now those users will be able to share photos, according to an exclusive report from Fast Company.

Later today, Hipstamatic will announce a partnership with Instagram that allows photos taken on the camera app, to be ported into Instagram’s network with just a kick. This will be the first time Instagram has opened up the company’s API to third parties.

On Hipstamatic, users will now have the option to push a photo to Instagram, just as they would with Foursquare and Twitter. Best of all, users can auto-tag the photo with a particular lens, film and flash, and add comments too.

On Instagram, these photos will appear with attribution: Taken with Hipstamatic.

Instagram and Hipstamatic Join Forces

According to Hipstamatic cofounder and CEO Lucas Buick:

“When we launched, it was all about Facebook and Flickr and Twitter, and now we’re seeing a huge shift in our user base toward Instagram. We’ve never been a social networking company, but we clearly benefit from social networks. So this will be the first app outside of Instagram that lets you into their network. That’s pretty cool for us.”

Prior to this announcement, Instagram photos could be pushed out to other social networks (like Facebook, Twitter, Google+ and Flickr). However, users couldn’t actually push photos to the service using a a different app.

Says Instagram CEO and cofounder:

“Really it all comes down to this: People post Hipstamatic photos on Instagram all the time, and we just want to make that experience easier. It’s a step in the direction that we’re testing out. We’ve been very careful about making sure that Instagram photos are about what’s happening right now in your life, and we want to allow for more of those photos to end up on Instagram regardless of where they’re taken.”

This is huge news for fans of both apps. Launched in October 2010, Instagram has nearly 27 million users; Hipstamatic launched in December 2009 and has four million users. Both apps were awarded Apple’s iPhone app of the year award; Hipstamatic in 2010 and Instagram in 2011.

The changes announced today should soon be reflected in app updates for both Instagram (Free) and Hipstamatic ($1.99). Both apps are available now in the App Store.