In order to not burn things up, you will need to allow the low speed circuit to brng the fan up to speed, then kick it over to the high speed that shuts off the low speed from there. If you don't follow that, you'll burn some wires and windings up just going straight to high speed because of the amount of current being drawn.
If your that concerned about using the high speed fan all the time because your car is overheating, then I would suggest you start looking for the problem as to why its overheating in the first place.
Personally, I hate the idea of using manual switches to kick on the fans, because sure as shootin, there will be that 1 time where your staring at that billboard sign with a girl with no clothes and your motor (the CAR son, not u) is overheating, then you will get to know that sign real good as you wait for the tow truck.
There's a careful balance as to what the optimal temperature the engine needs to be at or cooking the sucker, because running the engine too cold is also inefficient, and will lead to power loss as well.
Dave Dalke posted awhile back about the optimal op temp for these motors and it seems to be had by running a 180 tstat WITH A CHIP TO LOWER THE FAN ON TEMPS...... again I will say, WITH A CHIP TO LOWER THE FAN ON TEMPS......
Its been proven many times that just throwing a 180 tstat in with the stock fan on temps can cause the motor to run hotter.
If your car is overheating, I'd track down as to why. With the stock tstat, fan on temps, good coolant etc I can romp on the car and she doesn't overheat even in 110 deg weather.