Paul I have never strung squash rackets so I am not sure. If I were to string any racket pulling two strings I would use the 10% pre-stretch function. Do you think that may equalize the tension on the untensioned string?
Although I guess first I would try to measure the tension difference in the two strings to see what happens when double pulling is done.
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.