Use existing serial field with same counter across content types how do I use existing Serial field in other content types with same counter db table? 
Have 2 CCKs, cck-A has a serial field (auto increment) (ParticipantNumber), added same ParticipantNumber serial field (add existing field) in cck-B.
But the serial field value for both CCKs have independent auto incremental value.

How do I make the ParticipantNumber values be incremental (and Unique) across cck A and B? (just like node nid)
Instead of Serial type, I added Integer field, and added the same field (add existing field) in other content type as well. And In the hook_entity_presave, I query for the MAX(field_value), increment and assign for the current node/entity.