Double Majoring in Music Performance and Undecided

Hello all,
I have a question to those who have knowledge of double majoring.
I am planning on double majoring in music performance and something else which will probably be decided when I discover something that appeals to me. So my question is: If I get accepted into a college with almost a full ride for my music degree but get rejected on my other degree, is it impossible to double major? I am asking this question because my friend recently got into the Northwestern school of music but got rejected in the engineering category. I don’t understand if that means that you can only pursue music in Northwestern or not. My friend was really wanting to double major so I don’t know. Any advice or information would be great thanks!