First of all, a good rule of thumb to adopt is never use the system scope and system path to pull in dependencies. In my experience there's always a better way :-)
If Project_2 depends on Project_1, then first install it's jar into the local repository:
cd Project_1
mvn clean install
Watch the output you'll discover the jar is placed somewhere under the following directory:
$HOME/.m2/repository
Once this is done the jar will be available as a normal dependency to the second build
cd Project_2
mvn clean compile
The local repository ensures the projects are now decoupled from each other. Assuming you're using snapshot revisions of Project_1, the Project_2 build will always retrieve the latest revision built and tested.
Update
You should use a Maven repository manager to share jars between machines/teams. Recommendations are contained in the following answer:
Share jar of the module with another team
How to configure Maven to use a repository like Nexus is described in it's documentation.
As described in the deploy plugin documentation you'll need to add a distributionManagement section to your POM (detailing the repository URL) and then upload the project's jar to your repository as follows:
cd Project_1
mvn clean deploy