Making twiced baked potatoes and serving the next day, at what step to stop?

You can make them all the way through the final baking. Assemble them fully then place in the fridge to cool thoroughly. Wrap them in foil individually or put them all in one oven-proof pan. If you are going to have them for dinner, take them out in the morning, place them in the fridge to thaw during the day. Before putting them in the oven for the final bake - 'fluff' the filling a bit to prevent what betteirene said can happen (dense). Top with a little more cheese at the end of the baking and enjoy!

My rule of thumb: If you see it in the frozen food aisle, you can do it at home. They'll be great if you prepare them through to the final baking; refrigerate them and finish them off before serving, giving yourself an extra 10 or 15 minutes.

I've found that if I finish them all the way and then reheat them, they get browner than I want them to be, and they also lose some of their fluffiness and their height--they still taste good, but are denser.

You got it right, I've done it that way many times and they always come out fine. The "mix" does take a little longer to heat through than it normally would though.