The needles interact to a point..
If the high speed needle is already too lean from the low needle being too rich, leaning the low needle will adversely affect the high speed needle...A Walbro will run with the low needle way too rich if the high speed needle is almost closed, been there...
If the high speed needle is correctly adjusted the low speed needle will have very little effect on the high speed needle..Correct adjustment of the high speed needle is a few hundred rpm short of the highest rpm...
If you start with both needles open 2 turns and set the high needle first there will be no adverse effect from leaning the low speed needle...The low speed needle is correct when the transition from idle is good, just richer from the point where the engine dies when the throttle is opened...
You can NOT harm the engine adjusting the needles on the ground...If you get the high speed too lean the engine will just die...If too rich it will blubber and not run as fast as it should...