Can two people be charged and both found guilty of possession of one syringe?

I recently met with an ex-boyfriend to discuss the details of our break up. We were sitting in the car talking and a county police car pulled up to us. We were both asked to exit the car and provide identification. The ex-boyfriend had a warrant so they searched his car. He had a syringe in his possession and stuck it in the seat of the car. We were both arrested and charged with illegal possession of one syringe. My ex admitted it was his, has signed an affidavit and written to my judge and prosecutor to admit that I didn't have any knowledge of the syringe. My charges have not been dropped.