So I've been accepted by two departments at the same university for a PhD program. Department 1 offered me a TA, and Department 2 offered me an RA with a small fellowship. I would prefer to study in Department 1. But, would it be okay for me to ask Department 1 if I can enroll with them, but accept the RA from Department 2? And thus do research in Department 2 while studying in Department 1?