Create average rank from two disparate but overlapping ranking lists

I can't figure out how to create an average rank from two lists of baseball prospect rankings. Importantly, one list has 600 players and the other has 500 players, and while something like 300-400 players appear on both lists, many appear on only one because there is such a wide range of opinions among the two scouts, especially after you get past the top 100 or so. There are guys ranked 115th on one that don't even make the other list. So I can't just sort by name and create an average based on position.