Trimesh to trimesh collision fails to call the callback function for either trimesh.
Probably need this
if (!Callback((dxTriMesh)g1, g2, id1) || !Callback((dxTriMesh)g2, g1, id2)) continue;
at line 200 in collision_trimesh_trimesh.cpp (V0.8) to get it working w/the OPCODE collider. I'm unsure if the GIMPACT collider calls the callback.