Can I merge two users

Yes… although you have to remove the user that will be “loosing the posts”. You can add them again later (manually)

1. Go to Users > Authors and Users

2. Mark the user you want to delete and select “Delete selected users” at the bottom of the users list.

3. Select “Attribute posts and links to… [the user you want to move the posts to].

I hope that this helps. You will want to Backup your database and complete site before you do this just in case something goes wrong.