expression
duplex; two-family house
A building containing two separate dwelling units, often side-by-side or one above the other. Used in real estate and everyday conversation.
この地域には二軒建ての家が多い。
There are many duplex houses in this area.
二軒建ての片方を借りている。
I'm renting one side of a duplex.
長屋 is a traditional row house with multiple units in a line, often sharing walls; 二軒建て specifically refers to a two-unit building.
Compound of 二軒 (two houses) + 建て (building style). The exact historical derivation is uncertain, but the meaning is transparent from its components.