if there are two solutions of weak acid which have dissociated to a different extent, would the addition of more H+ affect the one that is less dissociated more substantially than the one with many H+ already in solution?
Well, considering neither are strong acids, the addition of H+ would have a greater effect the one that is less dissociated. Consider the dissociation of the following weak acids:
HF <---> H+ + F- Ka = 6.8 x 10^-4
HCN <---> H+ + CN- Ka = 1.8 x 10^-5
As you can see, the HCN dissociates to a lesser extent than the HF (i.e., HCN produces fewer H+ ions in solution than HF). Thus, a given amount of added H+ would much more significantly drive the equilibrium of HCN to the left than the equilibrium of HF.