It is something totaly different, they´r not slaves but wrong species brood. Slaves are something different, you call it slavery when for example Queen a) (here it could be Formica sanguinea) takes brood and workers from queen b (here it could be nearly every random Serviformica). Parasitic colony founding is when queen a) (for example Formica rufa) gets in the nest and kills queen b) and sometimes most of the workers and lives in the nest and is feeden by the workers of queen b), which die and are replaced by the workers of queen b).
In the case of Lasius niger and Lasius flavus it is NO slavery. Most of the Lasius would take random Lasius brood and some don´t kill the other spec. afterwards but sometimes they do.
Keeping them together means that you have a queen of both spec..





