A true double frame rail is a spine that protects the car from torsional twist and flex - exactly the loads placed on a car by big inch nitrous or blown engines with 1.80 first gear ratios, 4.10 final drives, and 35" tires. A true double rail car is not a single rail car with an arbitrary add-on. Once in place, the upper rail sees a lot of force if installed properly. Since cars designed to run a second frame rail will often juggle cross member heights, widths of various spans, floor material, etc., it’s important to do an update with an open mind and understand what the intent of the car is.
On high horse power cars, build double frame rails right to the front. Why? Because it ties the car in better if you go all the way to the front. Definitely tie it into the mid-mount area too, but it is better to go all the way to the front.