Hello Li; If the colony expanded into a second nest and continued to occupy the first one, the species is "polydomus". That is, the colony occupies two to several nests as the colony grows. One advantage to the colony is that it reduces the average commuting distance that foragers have to travel in their daily movements. This is a common life history trait. The genus Camponotus is an example. The behavior reduces the time workers spend travelling between food sources and the nest which reduces exposure to predators and the other hazards of life outside the nest. Use that term in a search and you will find many papers. Here is one example.

Article Polydomy enhances foraging performance in ant colonies

Best regards, Jim Des Lauriers