I went back to the Trachymyrmex septentrionalis nest the day after observing Solenopsis xyloni easily entering the nest. There were no S. xyloni there which means the two species are not nesting together. What S. xyloni was doing is still a mystery. There are other smaller species in the Solenopsis genus referred to as thief ants (S. molesta, for instance). It may be that S. xyloni was a thief but I did not observe this thieving behavior.