Your question is very broad--if you asked a more narrow and pointed question; then, you might get a more helpful response.
External femur lengthening is a safe and viable option for many people. The problem with the technique is that it is very challenging to lengthen both femurs at the same time. Some on this forum (i.e., "Pope") successfully managed the procedure.
Bilateral external femurs are challenging because you are essentially bed-ridden throughout the procedure. Why are you bed-ridden? Because the frames around your thighs prevent you from using a wheelchair. You cannot really scoot forward in a wheelchair to get your backside on the seat and still use the wheelchair.
As a result, most folks who do external bilateral femurs stay in a hospital throughout the procedure.
Otherwise, you can do one leg at a time. I am not fond of cross-lengthening or doing one leg at a time because you can never be certain that both legs will cooperate so that you get equal length. In contrast, when you are doing both legs at the same time, if one leg has a problem, you can stop lengthening the other leg until the problem is resolved.
Merry Christmas!