There is still a fair amount of debate over this question. Unfortunately for young athletes, all we have to go on are adult and animal studies. The standard treatment in adults with a combined ACL-MCL injury is nonoperative treatment for the MCL tear (bracing). Arthroscopic reconstruction of the ACL is done later.
This order of treatment comes from the results of studies showing better motion and strength when treatment is done in stages compared to patients who had both repairs done at once. On the other hand, some surgeons advise early ACL repair. They argue that when the ACL is weak or damaged, then the MCL can't heal as quickly.
Most of the studies are done on adults. Children and adolescents may have a different response to the timing of treatment. There aren't very many studies in this age group to help guide treatment strategy.
A recent report from Children's Hospital of Philadelphia (CHOP) showed equal results between two groups of adolescent athletes. Group 1 had an isolated ACL tear. Group 2 had both a torn MCL and ACL. Group 1 had ACL reconstruction. Group 2 wore a special brace for the MCL for six weeks before having the ACL repaired.
Results of the two groups were the same. All athletes were able to go back to their previous level of sports play after surgery and aggressive rehab. A similar study needs to be done with MCL and ACL repair at the same time. For now, we don't have results for treatment done at the same time.