Share a static class Helper ? , rethink your design to be more "stateless" ?
To share "running-code" please check this answer "Can I execute code from another file in a T4 .tt template for code generation?"
If you want to share code with your regular C# project, that is possible, but you need to build the shared code into an assembly you can reference. It's not possible to just use the <#@ include #> directive to pull in a .cs file directly as the directives don't nest inside control or class feature blocks. 'Quoted From the above Answer'