  We discuss the calculation of two-loop helicity amplitudes for quark-quark
scattering in QCD and four-gluino scattering in N=1 supersymmetric Yang-Mills
theory. We study the dependence of the results on different variants of
dimensional regularization. In particular, we consider the 't Hooft-Veltman and
four-dimensional helicity (FDH) schemes. We also discuss ambiguities in
continuing the Dirac algebra to D dimensions. For the four-gluino case, once
the infrared divergent part of the amplitude is subtracted, the finite
remainder is free of these ambiguities.
