Merging photo libraries in Macmini

Some time ago I changed the location of my photos library to my Macmini. At some stage I also created another library in my macmini and I now wish to merge the two. I read online this can be done using icloud and have increased my storage capacity accordingly in advance but can't see how to export the libraries to my icloud account. As a starting point I unchecked the photos box in my icloud settings even though I now realise photos were not being backed up to the cloud. Once I can merge the two libraries I would also like to back up the single library (in my macmini) to the cloud each day.