How do I connect 2 iCloud accounts to a single iCloud Photo Library
You need to have an iCloud id just for photos - Login on both devices using a (perhaps new) iCloud id which your photos will 'live' in. Turn off mail, contacts, calendars and reminders and notes on both devices for this shared iCloud id. So from now on, to apple's iCloud servers, it appears both iPhones are owned by the same "person".

After this, set up your mail id on your iPhone and your wife's mail on hers so you have access to your individual email accounts (you can also active contacts, calendars and reminders for this account so they appear on your respective iPhones).

Note: You will be sharing the same iCloud photo library, but also the same iCloud Safari passwords and wallet - but if you are sharing with one another all photos taken from your iPhones then this is (perhaps) a moot point...