An Overview of Maximal Unitarity at Two Loops
  We discuss the extension of the maximal-unitarity method to two loops, focusing on the example of the planar double box. Maximal cuts are reinterpreted as contour integrals, with the choice of contour fixed by the requirement that integrals of total derivatives vanish on it. The resulting formulae, like their one-loop counterparts, can be applied either analytically or numerically. 
Henrik Johansson, David A. Kosower, Kasper J. Larsen
hep-th