How to merge rendered textures (color/depth) to Unreal's frame?

I have a situation where I got DirectX textures (color and depth, sparse) from another app and I am looking for a way to merge them with Unreal’s frames. I already got shared textures (and the FRHITexture2D objects from them) and custom shader (in PostColorResolveCallback).

However, as far as I am aware I can not read from the same SceneColorTexture/SceneDepthTexture if I want to use them as render target. Copying the textures and using the copies as input works, but doesn’t seem ideal.

I am looking for any ideas how this could be improved (eg. how to swap the render targets, if possible).