It is available in ECC and not earlier releases like 46c.
If you go to IMG and search for ‘concurrent’, you will find a few steps where it is required to configure for this type of employment. (eg. Actions for Concurrent Employment, Activate Concurrent Employment for Personnel Administration, Assign Concurrent Employment Grouping Values to Registration Numbers, etc.)
For each step, you will find hyper text when you click the ‘hand’ button (when you put the cursor on it ‘it shows up text ‘IMG activity documentation’). This will guide you to do the config for the step.
For example under Personnel Mamagement->Personnel Admin->Personnel Admin for concurrent employment – you will use the “CCURE MAINS” switch where CCURE is the Group and MAINS is Semantic Abbrev. Under value – enter the start for implementing this concurrent employment in the system. You usually put the implementation date of the ECC version as the value.
You need to do all the IMG activities. Then test the same and change where you need or add any required user exits or BAPIs.
Hope this helps. Best of luck!