In main process you would have to
- drag a abstract activity (process design mode in workbench).
- Define that abstract activity.
- Search for your 2nd process and select it.
- Your 2nd Process is ready to be used in Main process.
You can make input/output variables for 2nd process so that you can pass/retrieve values from it in the 1st Main process.
Note: Make sure that application, to which the 2nd process belongs, should be deployed.
Hope it helps.
Thanks,
Abhinav