I'd be a little concerned that the extra weight on you ankles would actually put a greater load on your knees and encourage you to reduce your cadence although maybe I just made that up. You don't want to embarrass girlfriend by going overboard either.

I recommend using a lower gear that forces you to pedal quickly and work on pedalling smoothly. This is good exercise. I assume you do some solo rides. I don't think you should do all your training with your girlfriend.

Regards, Anthony