Is riding a stationary bike with knee tendonitis in both knees good for rehab?

Generally yes: For most "soft tissue" injuries (muscle, tendon, or ligament), non-impact activities can be very beneficial for rehabilitation and healing. However, accurate identification of the injury is important to develop the proper therapy plan. This is particularly important for more serious injuries. See you family doctor, orthopedist, or physiatrist for a referral to a physical therapist if necessary.

Ok: It should be fine as long as you start with low or even no resistance on the bike and exercise with comfort. If your knees are screaming at you during the exercise or if you have prolonged (greater than an hour) soreness after exercise, you are doing too much. Do not increase the resistance or the duration by more than 10% every couple of weeks and you should be fine. If not talk with pt or doc.