When you see guys selecting their Free Agents in here, they either have a duo that was the only one available during those two years OR they selected the last one available in that grouping.
If three duos are available during any two-year stretch (i.e., 1921-1922, etc.), and all three are selected by someone, then whoever selected the third one gets to choose their Free Agents first. Then it goes to the guy who chose his second, and so on...