Near the beginning of the floor file where all the dxt hex values are located you have to double the values to get double the resolution.

In the image above the value is already doubled from 00020002 to 00040004 so it also doubles the resolution from 512 to 1024.
So in the 2kxmodtool it looks like this:

As you can see the image size able to be imported has doubled to 1024 but this is where the other problem begins:

I think because the images are so large, they are leaking into each other's space within the file but I can't be sure.