The question was "Is it worth maintaining 2 sets of rules if many or most of the rules are the same?".
I haven't had much time for wargame stuff over the last few days due to some domestic issues taking priority. (Both heating and running water appear to be thought important by some members of the household) However, despite the expense, effort and emotional stress, I have taken a bit of time here and there to continue rebasing my 1812 Americans from too small flex steel bottoms to washers for greater stability and to match the British and I've had time to consider the question.
In short the flip side of the question is "What is gained and lost by trying to write one set of rules to handle two different styles of game?" Without going into details both sides lost something while gaining mere convenience. In particular what would normally be MacDuff games lost much of their OSW and toy soldier appeal while being too small to make good Hoft games while the HofT games lost some of their simplicity and smooth operation without gaining flavour. In particular the simple difference in troop quality was lost.
In order to reduce my own confusion I am going to share as many aspects of the rules as possible. For example the Order system is easy, effective and will work just as well with both systems so it will be shared. Likewise while 1 rule set will have one die for so many figures and the other will have so many dice per stand, the scores to hit and modifiers can be the same. Morale and the effect of hits on the other hand will have to be slightly different to get the appropriate feel, It will take a while to harmonize the fiddly bits and charts in HofT to match MacDuff but MacDuff is ready for a spin and feeling better than ever.
One of the bugbears that has been haunting me for the last 5 years is the question of matching historical War of 1812 to game units and maintaining scale without degrading the game as a game. I'm happy to announce that I've given up and admitted failure. The rules will allow you to try since what constitutes a unit is not fixed but I am falling back on my original OSW inspiration and will focus on about playing games and letting the armies build their own history. I've also decide to stop considering selling my original chunky units and redoing them just as the armies are coming together. Maybe I can build the missing Dragoons for both sides instead. .