Can I Submit To More Than One Role For The Same Breakdown?

No - you would need to select your number one role choice, and submit to that role. After you have submitted, no other submissions on that breakdown can be made.

As Casting may receive hundreds of submissions, they prefer each actor to be submitted only once per breakdown. Casting can choose to have you read for more than one role, or move you to a different role as they see fit. 

You can also always use the note option to let Casting know other roles on the project you believe to be suited for.