What do you think might be limiting flexion? Is the patella freely mobile? Is there a lot of swelling, and what is its quality? Is the quads mechanism tight? How much range did they have pre-op and at time of surgery?
I would nomally mobilise the patella, and massage to soften and release soft tissue edema and adhesions where present. Contract/relax techniques can be useful and auto-assisted flexion eg with foot up on a step and slow sustained stretches.
I really dislike applying passive push to gain knee flexion as the tendency is for the patient to resist due to pain and apprehension. Much better if they are able to gain this themselves. A static bike is good, but probably not practical if neither knee flexes past 100 degrees!
I always tend to discourage bilateral surgeries. I'm sure if patients knew what they were letting themselves in for they would think twice. The rehab period is almost invariably prolonged and most people I know admit afterwards that it would have been better to get one good joint before starting the next.