The patch generated by Synthesis for Immersive Equipment Display to generate mirrored sheathed weapons is set up incorrectly. It only generates an esp called Immersive Equipment Display - Mesh Patch.esp, which does nothing on its own. In Synthesis options for the patch, set the output folder to be the same folder as where the .esp is outputted and you will get a meshes folder full of mirrors, which fixes the sheaths for dual wielding.
The patch generated by Synthesis for Immersive Equipment Display to generate mirrored sheathed weapons is set up incorrectly. It only generates an esp called Immersive Equipment Display - Mesh Patch.esp, which does nothing on its own. In Synthesis options for the patch, set the output folder to be the same folder as where the .esp is outputted and you will get a meshes folder full of mirrors, which fixes the sheaths for dual wielding.