The custom or practice of having more than one husband at same time

One word substitution is Polyandry.

Polygyny: polygamy in which a man has more than one wife.
Polyphony: the style of simultaneously combining a number of parts, each forming an individual melody and harmonizing with each other.
Polyandry: polygamy in which a woman has more than one husband.
Polychromy: the art of painting in several colours, especially as applied to ancient pottery, sculpture, and architecture.