Can I effectively take Bactrim (sulfamethoxazole and trimethoprim) for staph and Amoxicillan for sinus infection at the same time?

Yes but unnecessary. There are few bacteria (in the sinus or anywhere else) treated with amoxicillin that are not equally well treated with sulfa-trimethoprim (Bactrim). And taking both will increase the risk of side effects, especially diarrhea or other gastrointestinal symptoms. Probably OK to stop the amoxicillin, but first check with your doctor.