You can possibly reach 6-7cm with internal femurs. See how your body responds and listen to your doctor's advice.
Splitting the surgery is safer, in terms of smaller lengths, but trust me once you recover from your initial surgery, chances are that you will not want to break your legs again and return to your LL condition!
Pre-op, I was adamant with 12cm by splitting my femurs and tibias. However, after achieving around 8cm on my femurs, I have decided not to undergo another LL surgery on my tibias, even though I'm still below average. I honestly don't want to go back to the state I was in during LL. It's a very difficult process and people that have actually done the surgery realise the tedious and painful nature of LL. I encourage you to only think about doing it once with a realistic target and then maybe think about a second surgery if you're still unsatisfied after the first surgery. Nevertheless, accepting your current height and doing no surgery is the best and safest advice.
Regarding the measurements you have provided, it would be best to see a doctor and ask him to measure your legs or take an x-ray. My pre-op femurs were around 41.7cm and tibias were around 37cm and I'm much shorter than you.
I reckon 6-7cm on the femurs is realistic and should not cause any obvious disproportion. You can always conceal this by wearing shoes to make your tibias appear longer.