Take directly from
Early Action FAQ | Application to Yale College | Freshmen | Office of Undergraduate Admissions :
"Q. I am applying to Yale under the Single Choice Early Action program. Can you explain the rules about applying to other colleges under their various early notification programs?
A. If you are an applicant to Yale under the Single Choice Early Action program, you may also apply to another institution’s early admissions program under the following conditions:
1. You may apply to a public institution in your home state, even if you will receive an admissions decision prior to January 1. (That decision must be non-binding, meaning that you have until May 1 to respond.)
2. You may apply to a public university's "rolling" admission program if the program is non-binding.
3. You may apply to a college with an early deadline for scholarship awards, but only if the notification of admission occurs after January 1.
4. Finally, you may apply to another college's "Round 2 Early Decision" program, but only if notification is after January 1. If you are admitted through another college's "Round 2 Early Decision" program, you must withdraw your application from Yale."
Hope that helps.