I think the normals can only face one direction. What TwoFace will do is duplicate the verts, reverse the normals and perhaps offset. The size of your mesh will double. You may get better efficiency if you select the area where the inside will show - duplicate just these verts - and do what it takes to reverse these normals. This would probably need be done in your modeler. I use RDS and it does not give as much control over normals as I would like. The grouping tool in Poser can save you if the verts to be reversed are assigned to their own group. Once this is exported from Poser the group can be turned into a material or removed.