The following procedure for separating twins is reported in "Structural Basis for Double-Stranded RNA Processing by Dicer", Ian J. MacRae, Kaihong Zhou, Fei Li, Adrian Repic, Angela N. Brooks, W. Zacheus Cande, Paul D. Adams, Jennifer A. Doudna. Science 13 January 2006: Vol. 311. no. 5758, pp. 195 - 198.
Crystals of the protein Dicer grown at 18°C by vapor diffusion in hanging drops composed of equal volumes of protein solution (9 mg/ml) and reservoir solution (28% PEG-400, 0.1 M MgCl2, 0.1 M NaCl, 5 mM TCEP, 1 mM DTT and 0.1 M MES, pH 6.5). Resulting crystals were pseudo-merohedrally twinned, producing an apparent spacegroup of P4222. Substituting MgCl2 with MnCl2 resulted in crystals that were macroscopically twinned. Individual crystals, which belong to the spacegroup P21212, were extracted by soaking the largest twinned crystals in reservoir solution containing 15% PEG-400 to weaken the twinning lattice contacts, followed by gentle mechanical prodding with a cat whisker. The resulting crystal shards were transferred back into full strength reservoir solution, cooled to 4°C for 1 hour and then cryo-cooled by plunging into liquid N2.