How can I merge/combine two contacts into one?

First, pick the contact you want to keep, and the contact you want to delete.

Then, on the contact you want to delete:

1. Select all their tasks and right-click to "Link Item(s) to a Different Contact..."
       (and choose the contact you want to keep)
        
    
2. Select all their history and right-click to "Link Item(s) to a Different Contact..."
       (and choose the contact you want to keep)
       
    
3. Link all their donor account(s) to the one you want to keep (more info here).
         
    
4. Everything else (Notes, phone, email, mpd phase, etc.) you'll have to manually combine.

Finally, after you have merged everything over to the contact you want to keep, you can delete the other contact.