I agree that it would be unnecessary complication. Stock center throttle is perfectly adequate for 2 pilot operations in a Kitfox.
I have some experience with dual throttle in a Zenith 750 which had a center joystick. O200 Continental. Worked ok except center stick radio work was awkward. Friction adjustment on the opposite throttle worried me a bit that the other guy might lock it down leaving me in pickle but it never happened
I am not an instructor but I do occasionally fly from the right seat. I am more comfortable on the left. Not that I feel I will try to flare with the throttle or power up with the stick. There are numerous tactile differences in the throttle and the stick that for me keeps that from happening. The issue for me is that the visual cues for runway alignment are different.
Bottom line for me is that the stock setup is the perfect compromise of access from both seats, simplicity, light weight, number of examples flown, universality (think C172).