Can I merge two cases?

The short answer is: No.

Case notes can be transferred from one case to another (if your user role has permission to do that).

Documents would have to be downloaded from the one case, then uploaded to the other.

Timeslips can be edited to change the case they are linked to (subject to all the rules that prevent editing timeslips after posting, that have a Date of Service outside the allowed date range, etc.).