Take this with a grain of salt. And use caution as this is only how im reading the wiring diagram.
There is a yellow wire coming from switch 1 to the first solenoid and splits off to switch 2. This is why you have to have switch 1 on before you can activate switch 2.
In******* theory******* if you replace the yellow wire from switch 2 with 12v positive. It *****should***** act independently. You would need to make sure you dont back charge the yellow wire as it will trigger the 1st solenoid.
Again this is just what i see from the drawings it may take more then that. It might not.