After quite alot of discussion, here is how we concluded that cases should be merged:

Say "Case 1" and "Case 2" need to be merged:

 Case 1

• Follow-up 1A
• Follow-up 1B
• Follow-up 1C

 Case 2

• Follow-up 2A
• Follow-up 2B

Then we start by creating a new case:

 Case 3

After which we add new entries in the follow-up tables of the merged cases:

 Case 1

• ...
• MERGED-TO 3

 Case 2

• ...
• MERGED-TO 3

Now we can indicate in the new case that there was a merge, so that when viewing the new case, we can still view the previous follow-up information of both cases:

 Case 3

• JOIN-FROM 1
• JOIN-FROM 2

After that, it’s as usual, we can add follow-up information to our new case:

 Case 3

• JOIN-FROM 1
• JOIN-FROM 2
• Follow-up 3A

When viewing "Case 3", the history could show the follow-up from the previous cases (while clearly indicating that there was a merge).

When viewing "Case 1" or "Case 2", the case overview should show that the current state of the case is "merged", and the most recent follow-up should include a link towards "Case 3".