Can I create the Advent Calendar for two kids

Yes, in theory, it's possible. If 12 characters are enough for both names to be printed on the poster then you can do it for two kids. The space for adding names is limited however and cannot be changed. 

Or, maybe you'd like to give each child their own calendar? If you use the code YAY15 at checkout, you'll save 15% if you order more than one product.