Combining and clipping DEMs

I have two DEMs for the terrain near the Golden Gate Bridge in San Francisco, one for the surface and one for the bathymetry under the Bay. I want to combine them, then clip them to a square. I’m a complete Blender noob and don’t know how to do this, but I’m guessing someone experienced could do this in under 5 minutes. My goal is a 3D model of the terrain around the Golden Gate Bridge that I can print. Thanks for any help.