Can I drive for two hours at at time 3 1/2 weeks post op from a BBL?

The best person to ask about this is your surgeon, as every surgeon has their own advice. Generally, I recommend that patients don't sit for at least 2 weeks after surgery. Sitting can damage the fat that has been transferred. If you have to sit, do so for very short period of times and alter your position so you don't put too much pressure in one area for too long.

Hello dear!!
Thanks for the question and provided information as well.
I recommend my patients to wait at least 10 days before flying/driving back to home, and you should not sit for more than 30 mins. every two hours, for the first 2 weeks, and always with the help of a boppy pillow. If you have any concerns, I suggest you to follow up close with your plastic surgeon.
Hope this is helpful. Good luck :)

I think that moving around a lot and not putting too much pressure on your buttocks is a good idea.  Sitting on the thighs and not the buttocks may be better.