First, let's look at the acceleration gradient over the positive mass shell. Using the shell theorem (which of course works just as well for negative as positive mass) we see that we can analyze the forces on the positive mass shell as due to a negative mass point source at the center. This means that we have a tidal gradient over the radial direction of the positive mass shell. The interior part will be accelerated outwards faster
than the exterior part, so the shell will be getting "squished". Looking at the effects of the positive mass itself, the shell theorem also applies so the interior part doesn't feel any force while the exterior feels an attraction due to the part of the positive mass shell inside its radius. It will also "squish" the shell. So the positive mass shell will expand along the angular directions and contract along the radial direction. So we'd expect to see redshift along two directions in space and a blueshift along the direction towards (and away from) the center. This badly fails observations.