noun
riding double; two people riding together
Typically used for two people on a bicycle, motorcycle, or similar vehicle. Often appears in prohibitions or rules.
自転車の二人乗りは禁止されています。
Riding double on a bicycle is prohibited.
彼らはバイクで二人乗りをして海へ行った。
They rode double on a motorcycle to the beach.
相乗り means sharing a ride (e.g., carpooling, sharing a taxi), not necessarily limited to two people and not focused on a single vehicle like a bike.
Compound of 二人 (two people) + 乗り (riding), from 乗る (to ride).