As you say, they cannot coexist unfortunately. You get that error because shadowMapCube is not a recognised name. It must be named shadowMap
The way around this is to pass the point light as a separate variable rather than via the setLight mechanism (which reads them into p3d_LightSource array). You can just pass the PointLight NodePath directly to a named shader input with setShaderInput, and use a different struct definition with cube shadow map to read in that shader input in the shader.