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Split repousse meshes does not work on anything else besides text. Kind of odd.
Split Repousse Meshes works with any set of more than one path/selection that don't have overlap it's just that when not working with Text Layers you have to first create the 3D Layer through normal Repousse. So for instance, make a selection, hold down Shift and make another selection that's completely separate from the first one. Go to 3D->Repousse->Current Selection, click OK and then you can select 3D->Repousse->Split Repousse Meshes