We agree with the reviewer that there is no easy answer to this problem. We now include a discussion of this in the introduction and cite Lynam and colleagues on this exact dilemma. In their paper, they discuss the Perils of Partialing (p.4, line 15), which is what you do when you control for the overlap. That is, one may be removing the reliable aspect of the self-report measures of aggression (when they are highly correlated) and remaining with residual error.
Creating groups based on a cluster analysis may have created unnecessary problems. In particular, and contrary to what the authors seem to believe, it is not possible to know whether differences between the two aggressive groups reflect differences in levels of relational aggression or in type of relational aggression (reactive only vs. combined), given that the combined group is obviously much more aggressive than the reactive-only group. A better strategy would have been to predict each type of aggression while controlling its overlap with the other type (i.e., include reactive aggression when predicting proactive aggression and vice-versa).