As someone who fenced for 3 years with saber, rapier, epee, and foil, its a silly idea. The only off-hand they ever used was a sword breaker, and that was a wildly niche subset. Longswords are absolutely not finesse, my god. There is no delicacy whatsoever with that weapon. Realism aside, there are entirely too many balance issues with this. Its like it is for a reason.

What is the problem you are solving? Does your proposed change solve the problem? Is your change feasible? What else will be affected by your change? Will your change impact revenue? Does your change align with the goals and strategies of the organizations (Larian, WotC)?