You can tell them what you just said, that you aren’t sure about the REA school but don’t want to give up the spot yet because you don’t know where you’ll get in.
WHY do you want to apply to the second school? Is there a program at that school you just found out about? How would you be ‘hogging’ the spot from another student if you apply RD? I can see the school not allowing you to apply ED2 because you’ve had your shot in an early round but you aren’t taking anyone’s chance in RD. Does your high school only allow one top 20 if the student applies RD?