Hi Giorgos,
you can connect the same source system as SAP-API source system and as ODP- SAP (Extractors). But you will need to give both source systems a different technical name.
E.g. BWDCLNT100 for the SAP-API source system and BWDODPS100 for the ODP-SAP source system connection.
If you use source system ID in your BW data model e.g. in a in multi system/client landscape, you can use the 2-digit source system IDs (0SOURSYSTEM).
You can assign for both source systems (example BWDCLNT100 and BWDODPS100) the same value for the two digit key (e.g. 'BD') in configuration "Assignment of Source Systems to Source System IDs" in RSA1 -> Tools.
Best regards,
Andreas