The relationship between the two is mutually supportive and works out well.
Literal
Both-parties [possessive-の] relationship [topic-は] give-and-take [with-で] convenient thing is.
持ちつ持たれつ is a set expression meaning 'give and take' or 'mutual support' — literally 'holding and being held.' The つ…つ pattern alternates active and passive forms of the same verb to express reciprocity. 都合がいい (convenient/works well) completes the picture.