gradle project dependencies

How do I add a gradle project as a dependency of another gradle project? I tried to add it like so:

dependencies {

    compile project(':cache_data')

}

But this seems to cause problems because I've already attached that project into the current project. I did this because I want to edit both projects.

I also tried to add 'cache_data' as a dependency using the Project Strcuture -> Modules window, but syncing gradle overwrites this.