Well, This question is just for design proposes.
2 clients ClientAand ClientB users can access to their office using their IDcard. This IDCard is saved in a DB using salt&pepper to create a user pseudonymization between the clients.
ClientA DB
Username String to be Hashed Hashed Value
ClientA UserIDCard1+E1F53135E559C253 72ae25495a7981c4062...
ClientA UserIDCard2+E1F53135E559C253 a52e4f1565c90f048f8...
ClientA UserIDCard3+E1F53135E559C253 59027bd9c8c8900d5c3...
ClientB DB
Username String to be Hashed Hashed Value
ClientB UserIDCard1+B7E459A02CB31F3C b4b6603abc67096754...
ClientB UserIDCard2+B7E459A02CB31F3C e99c7e7f1389e40cd3...
ClientB UserIDCard3+B7E459A02CB31F3C 16e78ad38eb1468edf...
That means if UserIDCard1 visits ClientA and then ClientB, the UserIDCard1 will be treated as a different user, which is good for pseudonymization (If an attacker gets access to DB) but bad if in the future Clients decide to merge ClientAB.
Is there a way to keep the pseudonymization across DBs but at the same time know if UserIDCard1 was in ClientA's building and ClientB's building? Or by definition Hash+Salt are NOT to be able to compare hashed values?
edit
The propose of these DBs is to get anonymous statistics about Client's users without compromising user's privacy across different DBs by storing their 'UserIDCard'.