@Sunshinepacific - Yes, my first year after the injury I used a DonJoy A22 knee brace for kiting, then from the next summer I am kiting
without the brace. I had suffered injury similar to yours - complete tear of ACL and MCL. No operation.
This year I have approximately 60 days kiting, mainly twintip, started with the strapless surfboard as well. I am not advanced rider - I do small jumps, backrolls,...basic stuff....nothing really dangerous for the knees. At the stage of learning foot swapping on surf board I have the impression that it is easier on the knees than the twintip riding. But wave riding could be really different.
As I wrote - I try to keep myself in a good shape and I am a lighter rider(~70kg). I also undergo MRI once a year, just to make sure there is no abnormally high wear in the knee joint. If I start having pain and difficulties doing my sports - I will undergo ACL recon.