How do I merge an iPhoto library on one MAC with my new photos library on my other MAC

You can't. What you need to do is use the paid version of iPhoto Library Manager to merge the two iPhoto libraries and then migrate the merged library to Photos.

Otherwise what you can do is open the iPhoto library package like this:

and copy the Masters folder to your Desktop and then into the open Photos window or onto the Photos icon in the Dock. This will get all of the iPhoto library originals into your Photos library but you will lose any edits, metadata added, projects, etc.

So if you can merge the two iPhoto libraries and go that route you'll be better off.

If you use iCloud Photo Library on both yo will have a merged library on all devices insulting iCloud.com after all of the uploads have completed

LN