This Unity project contains two examples of blending cubemaps and blit cubemaps. (Built-in and URP compatible)
1. Blend cubemaps using ReflectionProbe.BlendCubemap
You can blend two cubemaps into one render texture simply by using ReflectionProbe.BlendCubemap(src, dot, blend, target) provided by Unity.
2. Blit cubemaps using Graphics.Blit and Graphics.SetRenderTarget
If you need to blit cubemaps with a custom shader, you can use Graphics.Blit. You can assign a cubemap face to a render target using Graphics.SetRenderTarget.