I never anticipated this coming up prior to the start of this round, but here it is...
If a team that's available has 22 or fewer players/pitchers available to use, they wouldn't be able to roster a full team with the two Free Agents. In the case of this happening, I'll say just make the 25th player an "under 300K" guy (pitcher or position player) from within five years of either side of your duo's two-year stretch - and have it be from the same franchise. I wouldn't anticipate that player being called upon very much - some owners might even put them on permanent rest.