  We study the multiparticle factorization properties of two worldsheet
theories which--at tree-level--describe the scattering of massless particles in
four dimensions: the Berkovits-Witten twistor-string for N=4 super-Yang-Mills
coupled to N=4 conformal supergravity, and the Skinner twistor-string for N=8
supergravity. By considering these string-like theories, we can study
factorization at the level of the worldsheet before any Wick contractions or
integrals have been performed; this is much simpler than considering the
factorization properties of the amplitudes themselves. In Skinner's
twistor-string this entails the addition of worldsheet gravity as well as a
formalism that represents all external states in a manifestly symmetric way,
which we develop explicitly at genus zero. We confirm that the scattering
amplitudes of Skinner's theory, as well as the gauge theory amplitudes for the
planar sector of the Berkovits-Witten theory, factorize appropriately at genus
zero. In the non-planar sector, we find behavior indicative of conformal
gravity in the Berkovits-Witten twistor-string. We contrast factorization in
twistor-strings with the story in ordinary string theory, and also make some
remarks on higher genus factorization and disconnected prescriptions.
