1. Can one Mac have two iCloud accounts?
2. How do I set up a second iCloud account on my Mac?
3. What happens if you have two iCloud accounts?

Can one Mac have two iCloud accounts?

You cannot use two different Apple/iCloud accounts on the Mac itself. You can set up iCloud Library for each of your iCloud accounts, but they are separate libraries.

How do I set up a second iCloud account on my Mac?

Setting up a second iCloud accounts is easy. To create a new account from a Mac, launch System Preferences from the Dock, and then click the Internet Accounts option. From the Internet Accounts menu window, click the iCloud option.

What happens if you have two iCloud accounts?

All devices using that Apple ID share all messages, conversations, iCloud emails, contacts, calendars, phone logs etc. Not only albums, but ALL photos taken on whichever device will sync to both devices. Be careful not to delete anything though as once this is deleted on one device it will also be deleted on the other.