The high speed needle does in fact screw into the seat of the low speed one. The critical factor here is the taper of the needle and its relationship to the seat. It may run fine at high speed and poorly at idle or vice versa. The key is to get it to do both. Adjust the high speed first , then adjust the idle and then readjust the high speed. repeat as necessary until it runs right! It is a trial and error process.

Max