Double Spacelike Collinear Limits from Multi-Regge Kinematics

We study scattering amplitudes and form factors in planar N=4 super Yang-Mills theory in the limit where two pairs of gluons become collinear. We find that, when the virtualities of both collinear pairs are spacelike, the collinear factorization of the amplitude involves a generalized splitting amplitude that correlates the two collinear directions, confirming a recent proposal in the literature. Remarkably, we find that our generalized splitting amplitude agrees with the Bern-Dixon-Smirnov subtracted six-point amplitude in multi-Regge kinematics. The latter can be explicitly evaluated using integrability to all orders in the coupling. We also present compelling evidence for the universality of the generalized splitting amplitude by showing that the same function governs the double spacelike collinear limit of scattering amplitudes with up to eight particles and three loops and form factors with up to six particles and two loops.