同屋 is a verb-object compound (离合词) meaning 'to share a room with someone'. It commonly acts as a verb, and you can insert time expressions or measure words between the two characters, e.g., 同了两年屋 ('shared a room for two years'). Although colloquially it is also used as a noun meaning 'roommate', the dictionary classifies it as a separable verb, so pay attention to its grammatical behavior in formal contexts.