Is it possible to put multiple recipient email addresses on the same Thankbox?

You cannot send the same Thankbox to more than one person - it's intended for a single recipient. An alternative you might consider is to use Thankbox cloning to create copies of one card for multiple recipients.

Another alternative if you're not collecting cash in your Thankbox is the following: 

1. You can send it to one person and then have them create a share link to it. 
2. Then have them send the link to the others. Sharing is an option available to recipients from the secure link they receive to open their Thankbox - it'll be uniquely generated for that Thankbox and can be sent to anyone.

You can see an example of how sharing works at https://www.thankbox.com/sample