I tried using a gauge on the front and a sensor for electric fans in that rear port. It didn't work. It's the manifold design. Looking at the port it sure looked like it would. It would be interesting to try switching them for a test.
If the sending units are the same thread size you could switch the units to see if the gauge readings change. If they do not either the gauges are faulty (I would think it’s unlikely that both would be defective at the same time) or you truly have two different operating temps. Now whether that’s a thing or not will depend on if you have any other symptoms going on. I’m guessing not.