Maximal Unitarity at Two Loops
  We show how to compute the coefficients of the double box basis integrals in a massless four-point amplitude in terms of tree amplitudes. We show how to choose suitable multidimensional contours for performing the required cuts, and derive consistency equations from the requirement that integrals of total derivatives vanish. Our formulae for the coefficients can be used either analytically or numerically. 
David A. Kosower, Kasper J. Larsen
hep-th hep-ph