What is more dangerous?
In the first case we are overlengthening, which is associated with higher rate of complications
In the second case, we are smashing the bones in our legs but we are lengthening only to the "safe" limits
8cm femurs is probably safer. Assuming internals nails such as Stryde, femurs are much easier to lengthen due a number of reasons including:
- better blood supply
- only one bone per leg to lengthen
- bone thickness allows thicker nails to fit - easier to be weight bearing
- the knee is not touched during surgery
Lengthening the tibia involves breaking two bones per leg, the bones are thinner and more likely to fracture. The soft tissue is more resistant to lengthening often resulting in "ballerina" foot. The recovery time is long.
The combination of both surgeries even if done at different times doubles the surgical risks, greatly increases the risk of complications and increases the recovery time significantly. See the diaries of quad lengthening patients such as MyEvolution, OverrideYourGenetics and IamReady etc.