When cross lengthening, you lengthen 1 femur and 1 tibia and then alternate to the other 2.

How do the doctors make sure that if you lengthened 6cm on your left tibia that 3 months later you'll be able to lengthen your right tibia to the exact same amount?

Your legs start to feel tight at different times, your left tibia might be more relaxed and make it to 6.5cm and then 3 months later your right tibia can't make it past 5cm