Surely someone that has already pulled the trigger on the first code and got another in an email could test it out to see if both will work. You don't have to actually buy anything to see if it will let you use it.
Can't stack them, but yes you can use the email one even if you've already used the MLB one
edit: I tried generating another 20% off code with a second account and you cannot double use the codes from MLB, just tells you it was already redeemed.
Here's a code for someone who wants it and hasn't done MLB - 7G4BP739JF