Question

Can I merge the contents of two databases

Comments

In theory yes, but in practice a full merge is probably not worth the time and effort it would require. What is the actual problem you're trying to solve by doing this?

---

I believe a merge will require export of posts and user tables. Then import of these in the other database. When users are added to the new database, old userid in the posts table must be changed to the new one. Same should be done for categories if they are used.