Dear hherbol,
sorry for the late reply, but maybe this is interesting for others, too. It is not possible to calculate SOC constants in a 2c X2C calculation. This is not a technical limitation. In a 2c X2C calculation, SOC is included already in the orbitals, and SOC and dipole moments loosely spoken form the transition dipole moment -> which is automatically non-zero also for singlet-triplet excitations in that case.
Best regards