Living together as two people. Commonly refers to two people (spouses, a couple, or roommates) sharing a household.
結婚して二人暮らしを始めた。
We got married and started living together as a couple.
二人暮らしだと光熱費も安くなることが多い。
When two people live together, utility costs often become lower.
友達と二人暮らしをすることにした。
I decided to live with a friend as a pair.