Yes you could have both. An FM diagnosis does not preclude a RA diagnosis. Besides the fact that both disorders are often accompanied by alot of pain there are some differences in symptoms. RA is generally accompanied by localized pain and joint swelling. FM is generally characterized by widespread pain (and sometimes localized pain) without joint swelling. The hands and feet of FM patients can swell but the swelling isn't concentrated on the joints as occurs in RA. Several blood tests can be used to assist physicians in a RA diagnosis. Unfortunately the same cannot be said for FM.

That said neither disease is particularly easy to diagnose and some patients kind of toe the line between each. 

http://www.healthcentral.com/chronic-pain/fibromyalgia-treatment.html