Why don't you connect them directly to 12V, that way both will do max RPM possible?
Because if the controller doesnt see the fans the machine shuts down and will not hash. Honestly if you ut the PWM line and it is still not speeding up you most likely have a bad fan.
thats what i throught but same problem with both antminer? one i purchased in november last year and one i just received (i cut both cables but onlye front fan is working at full speed ) ..