I haven't seen any actual tests on it, but if you want to try to mitigate spauling, you would want to put the soft armor on the outside of the hard armor.
If you're talking about spalling in terms of fragments of the projectile splattering, I agree.
However, when you're talking high speed rifle velocity projectiles hitting steel, you have to be concerned about spalling on the other side from the armor itself, i.e. bits of metal driven from the body-facing side of the plate due to the transfer of energy through the plate.
Yeah, I guess I meant more related to distributing the energy of the impact to the body. I plan to get coated steel but the plates are meant to be used with trauma pads underneath (against the body). The question is, would the soft armor have the same effectiveness as the trauma pad in dispersing the impact?