If the tendon graft is taken from the same leg, then only one leg is affected. The patient can shift the weight off that leg during the early days after the surgery.
Repairing the knee with donor tissue from the other leg means both sides are affected. There have been a few cases reported of problems developing from overload of the donor side.
This is more likely during the first 24-hours when the patient is still under the influence of anesthesia and drugs to limit pain. Without complete sensation, the patient can put too much load on the donor leg. The result can be an avulsion fracture. The remaining (weakened) patellar tendon pulls away from the bone.
On the plus side, taking donor tissue from the other leg leaves less trauma to the reconstructed knee. Rehab can progress along much faster.
Most surgeons use donor tissue from the same side. Talk to your surgeon about his or her preferences and reasons for choosing one over the other.