Hi Maru, thanks for the quick reply.
I only have one hdri. The main problem I am struggling with is that i can't seem to make the backplate visible in the reflections of the windows. As you can see in the attached scene screenshot, i made the backplate very large, but i still only get the reflections of the hdri in the windows. The problem i face with using an extra hdri is that the reflections are very large/blurry. I want to have more control over the reflections, therefore i want to use the backplates.