Sum of Opponents SOS
Someone's opinion: Generally it's not a good idea to use SOSOS as secondary tie breaker because it is pretty much random and not a good tie-breaker.
Christoph Gerlach: There are different views on this. All tie breaker dependent on the performance of the opponents are more or less random (actually even others like CUSS are). But it's more likely for a stronger player to have more SOS and more SOSOS compared with a weaker player. If you need to break ties in your tournament, SOS/SOSOS are logical: SOS evaluates the average strength of the opponents, SOSOS evaluates the average strength of the opponents of the opponents. It's likely that the player reaching a specific score against the strongest opponents is the strongest among all players with his score. If you dislike the variance of SOS/SOSOS in your tournament, allow ties or play more rounds or select a higher McMahon-Bar (have less players in the top McMahon-Group).