The apps, Duo and Allo both are up for Registration but is not available for download. Though, some guys managed to extract the APKs of both the apps which are available for download from the source links below.
Well, those apks are of no use as they gets installed but the verification process of your phone does not get initiated. That may be due to the servers, which are currently not attached to the new messaging service.
So in order to see what’s inside someone need to jump over the verification process.
Similarly, a guy over the XDA managed to do this cracking and fixing part. He used XDA Module to use the App and eliminate the not working verification process on the apps.
Here’s the video he recorded about the Allo app:
So you can have full information over the XDA.