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.
Everyone has the right to be stupid... it's just that some people abuse the privilege