Most sets of string are not 40' but 40.5 to 42 feet long. If you are going to use 1 set of poly string for two racket's main don't cut the string before you start. Use a complete set with the short side as short as you need. When you finish and tie off the long side then cut it and you will have the longest possible string for your second racket.
Agassi had a similar problem when he used Kevlar that only came in 20' lengths and he was experimenting with a 20 main racket. He (his stringer) would string the center 18 mains with Kevlar and use the cross string for his outside mains.
I have the ability to drive people crazy, not sure if I was born with it or I learned it but I'm damn good at it.