Can you use two Easy Roam passes back to back?

Yes you should be able to activate another one after it. If the first one drops off the account, you should be able to add it again with it listed in Manage Plans and add ons → Travel with Easy Roam.

If you cannot get it added, you can post the issue here and a rep can be flagged to assist. 

To avoid any unnecessary charges, you may want to send out the text message to add the Easy Roam Travel Pass on the last day before it expires. https://www.koodomobile.com/en/help/europe-easy-roam-travel-passes

I had a pass for 7 days in Mexico, that pass has ended and I have tried to add another one but can’t seem to do it and I am currently being charged the daily amount which is expensive. How can I get a new one and renew it on a weekly basis as I am here for several weeks. The daily roaming fee is very expensive. Should I just get a local sim plan for data? 

Not so sure about that but maybe there a limit on consecutive roaming passes. Some carriers restrict extended roaming use, but Koodo doesn't clearly state this for Easy Roam Travel Passes. If you're stuck, you can try using a local SIM for data as a backup, but keep your Koodo line active for calls and texts.

What happens when you text the code to try to add the travel pass?  Do you get a reply? A confirmation?  An error message?