You can only bill it once per joint regardless of how many times they "stick" the joint. It is a single joint. He just went in from both sides.
This is the same idea as you can't bill it 2x if they aspirate and inject the same joint. It is one.
7. CPT codes 20600-20611 are a family of codes describing arthrocentesis for aspiration and/or injection of different sized joints or bursae with or without ultrasound guidance.
The UOS for each of these codes is a joint and its surrounding bursae, if any.
A provider/supplier shall not report more than one unit of service for arthrocentesis of anyone joint regardless of whether or not the physician also aspirates or injects one or more of its surrounding bursae. For example, if a physician performs arthrocentesis of the shoulder and 2 bursae of the same shoulder without ultrasound guidance, only 1 unit of service of CPT code 20610 may be reported.