Z3tA question about filter

I am just wondering what the filter 1/filter2 slider does in the osc section of the synth. When I have a wave and make the slider all the way down on, say filter 2 I would expect it to only be affected by the second filter, however when I change anything to do with the first filter it will change the sound of the wave as well. What does this slider do exactly? 
 
Thanks for reading