How to merge texture sets when exporting?

Hi, I hope this is the correct place to post this.

I am trying to import a flat mesh with 2 textures from SketchUp to Quixel, texture it and then export 1 single Albedo +1 Normal and +1 Roughness map.

It's a simple flat square measuring 7.5x7.5 meters. There are 2 textures on it - the asphalt, and the the white road paint. The road marks are modelled in 3D. They are meshes on their own as shown below. It's very important to have the road marks modelled like this, because they have a precise width and distance that's important for the project. 

SketchUp is very bad with UV mapping but I managed to properly import a UV mapped OBJ into Quixel, did my textures and here's the final result:

Now I'm trying to export what I see in the viewport as 1 single flat texture but it just doesn't work. I even tried "Flatten Mix" command but this is what I get in the end.

As you can see I get a separate asphalt texture with its 2 maps and a separate white paint texture with its own maps. I have no idea what to do at this point. Someone please help.