On embedded products of Grassmannians

Let $\Gamma'$ and $\Gamma$ be two Grassmannians. The standard embedding $\phi:\Gamma'\times\Gamma\to \bar{P}$ is obtained by combining the Pl\"ucker and Segre embeddings. Given a further embedding $\eta: \Gamma'\times\Gamma \to P'$, we find a sufficient condition for the existence of $\alpha\in Aut(\Gamma)$ and of a collineation $\psi: \bar{P} \to P'$ such that $\eta=({\rm id}_{\Gamma'}\times\alpha)\phi\psi$.