Likely, opacity settings are preventing one image from showing. To adjust the opacity of the image base layer on top, select the Base layer in the Contents window, and click the Layer tab in the Properties window. Adjust the Opacity for that layer (e.g. change it to 40%). As long as the Opacity is not 100%, you can see the other image base layer under it.
Surfer 8 and previous versions did not support opacity for layers or images. All images would be opaque. Therefore, whichever base layer was on top in the Object Manager is what is seen on the screen. If you are using Surfer 8 or a previous version, you can consider upgrading to the current version of Surfer.
Updated February 13, 2017