Hmm... that's kind of an odd one. How about this: tell me how many springs are used on each side of the window, and the weight of the sash. With that information, I will be able to determine which option is best.
We took out 1 spring on each side of the lower window. Because it's double hung, there should be 1 spring on each side of the upper window, too, correct? The entire lower window (sash) weighed 20.8 lbs when we took it out; it's 29 1/2" tall and 36" wide.
How would we fit 2 springs into the space? Originally, we only had 1 spring on each side of the lower sash. Thanks.
There should be plenty of room in the jamb for two. To insert it into the slot, just put it in the cut-out shoe first and slide the whole balance system in.