We extend the maximal unitarity method at two loops to double-box basis
integrals with up to three external massive legs. We use consistency equations
based on the requirement that integrals of total derivatives vanish. We obtain
unique formulae for the coefficients of the master double-box integrals. These
formulae can be used either analytically or numerically.Comment: 41 pages, 7 figures; small corrections, final journal versio