状態・変化N1
〜つ〜つ
now... now... / alternately... / mutually...
Structure
V-ます形（語幹）+ つ + V-ます形（語幹）+ つ
Explanation
V-masu-stem + つ + V-masu-stem + つ: two actions alternate or are mutual. E.g., 行きつ戻りつ, 持ちつ持たれつ.
Examples
彼は部屋を行きつ戻りつしながら考えていた。
He paced back and forth in the room, thinking.
夫婦は助け合い、持ちつ持たれつの関係だ。
The couple support each other in a give-and-take relationship.
Note
Mostly fixed pairs (行きつ戻りつ, 持ちつ持たれつ, 浮きつ沈みつ). A literary remnant.