What you can do as an alternative to directly managing it with git, is to separate the common code into its own Android Library Project. Then you would be able to add that library to the other two projects, giving you access to the same code base.
This would have you end up with three projects, and three repositories. Two of each for the separate projects, and one of each for the library project.
One thing to be aware of, however, is that if there is specific code for one project inside of the library, it will also affect the other project. Some of this could be handled by subclassing and overriding methods/properties as needed. Another way around this, as suggested by jul, would be to have branches for each project.
A nice benefit of doing it this way is that if you make a bug fix in the library that you found while working on one project, you'll be able to get that in the other for practically free.