A 12 cm increase in leg lenght would probably wreck his proportions and physique. I've been trying to figure out what a ll paient can expect post-op and most people tell me that even after one surgery they are never quite the same again. They can't walk like before because their legs are out of synch with their proportions, they can't run like they used to because their muscles are weaker and they never quite get rid of the pain or stifness never mind how they would recover after two surgeries.
I would like nothing more than to do 6 + 6 cm and be 190+ that would be the dream but I know I would regret it when my legs move like stilts, my flexibility is reduced and my arms and torso look too short for my height.
That being said perhaps some stifness and cancelled marathons is worth it to be taller, I think for many, and perhaps me, it is. Just be realistic and don't take chances just to get 3 or 4 cm extra.
Also don't compromise on safety if you get some sort of complication all the above could be ten times worse.
This is a very mature and realistic approach in my opinion. Keep in mind that each body is different as well. My room mate has a friend who lengthened 7.5CM on his femurs with Precise and Dr. Donghoon Lee right after finishing high school (at around age 19). He did it with his parents' money and this was able to do it that young. He started at 174 or 175CM. With six months of physical therapy after finishing lengthening, he claims to have made a 100% complete recovery. My room mate says he can run as before without any problems. I will gradually find out more information about this, but I don't want to seem too interested so that I don't accidentally reveal to him (and so he doesn't tell the world) that I am insecure about my own height and seeking LL myself.
I also believe that Shyshy's sprinting video shows a recovery near 100%. He claims to feel the same as before, just 7.5CM taller.
Some don't recover even 80%. It's a risk you have to be willing to take. If it his happens to you, the other medical procedures will probably allow you to recover even more, but they will be pricey if necessary, and you must be willing to accommodate for the fact that things could feasibly go wrong.
Doctors generally agree that people like us, who have relatively taller starting heights, generally handle lengthening better and can tolerate more lengthening. This is a theoretical rule that applies generally, but it won't by any means guarantee you a smooth LL journey.
I myself plan to do 4cm on tibiae and 5cm on femurs for a good 9CM gain that hopefully won't be too harsh on proportions. This should put me right around 188CM. My sitting height is 96CM and arm span 186CM, so I think I can look perfectly normal to 90+% of people. By lengthening both segments, the femur to tibia ratio is preserved and biomechanics shouldn't be significantly altered. Additionally, Dr. Birkholtz himself told me that lengthening up to ~5CM in one segment shouldn't significantly alter biomechanics in his opinion.
I would rather have occasional aches and slightly worse coordination and balance than live with the shame and jealousy of others that I often experience being my current height.