How is it impossible to measure? It has nothing to do with how his content quality ranks compared to other people. Since he is writing both the content under his name and the content under a different name, given that he doesn't know which one he's writing under until after he writes it, the quality should be at the same level (on average).
If the quality is the same but the points are different (between his real acct and anon acct), then you've found a bias.
OK, yes, it is possible as long as the other account never answers meta questions about this site or YC, and all meta-questions are eliminated in the comparison. It's still not worth doing, because there is a pg bias: people follow his comments and submissions and are more likely to vote them up because they are more likely to see them.
If the quality is the same but the points are different (between his real acct and anon acct), then you've found a bias.