As long as the two projects are deployed to Engines that share the same directory service, a process in one project can create an instance in a process in the other project using Dynamic Process Invocation.
For the process you'd like to invoke:
1. Create a Process Interface (right mouse click the process in the Project Navigator tab -> "Generate Process Interface"
2. Export the Process Interface process you just created
For the process you'd like to have invoke the subprocess in the other project:
1. Import the process interface you just exported.
2. Create either a Subflow or Process creation activity in the calling process
3. As you're adding the activity click "Related Process" -> check the "Dynamic Process Invocation" checkbox -> select the process interface process you just imported from the dropdown.
4. Map the variable(s) going in and out of the process you're calling.
