Jill,
They are 2 stand alone entities, although if you implement a quality system that encompasses all of the requirements of both, you could be audited to both at the same time and be accredited/registered to both as a combined "quality system"
I have no idea about the costs involved in such an endeavor, but you would need a registrar/auditor that is "allowed" to audit both entities.
Call a couple of registrars and see what the process would entail, they are looking for business and could be very helpful.
ASD...