The most important thing is to have twin themes. The cities may be similar in population, culture, historical events, geography, natural richness or any structure. The people may be similar in personality, looks, background and accomplishments, or any other commonality drawn up by the poet. Poems about similar birds, animals, forests and any thing in nature are other examples. Twin poem genre can create a lot of opportunities for poets who want to make their mark in the history of literature.
Poetry written by many famous poets provides opportunity to write twin poems.
If someone chose poems by famous English poets like Shakespeare, William Wordsworth, William Blake, Lord Byron, and also many modern poets, and then wrote twin poems for them, they can be read side by side. It also gives a challenge to the current poets and an outlet to their creative talents. Imagine your twin poem you would write for the immortal poets of literature. Twin poems are not limited to English. Any language of the world would have opportunities for twin poems.