Can I use two separate Creative Cloud accounts on the one Mac?

Short answer: YES

Long answer for anyone interested: Logged out of my personal freelance account.  Went to an office, was set up as one of their new users, so different login. Next day back home logged in with new 'work' account for the day and after a little downloading/syncing of files everything was there just like at the office. Next day back to freelance account, all fine.

When you log out of an account, the 'Creative Cloud Files' folder disappears momentarily. When you log back in with another account, it reappears with the Synced files from that account. This is the bit I was concerned would have to re-download, but as David__B said, they are still there on your HD, you just can't access them unless you're logged in. So the only wait time is if it has to sync new files you might have added while at the office, or that someone shared with you.

Well done Adobe, was a relief it all works smoothly.

Nancy is correct. There is a unqiue instance of the Creative Cloud files folder created for each account (Adobe ID) used, these are hidden by default but the appropriate one becomes active at sign-in. Here is a screen shot of my user directory for example with hidden files displayed.

Assuming you have one plan from work/school and a separate individual plan under a different Adobe ID (email), you will need to log-off your current account to access the 2nd one. 

You cannot access 2 different accounts at the same time.  

Files saved to your hard drive and/or Cloud storage remain there until you delete them. 

Hope that helps.