Basically, you should create a ZaphodHead setup with two monitors each and then use either xrandr or the display manager of the running desktop to clone each.
I was able to solve this - it wasn’t zaphodheads. I think I should have explained better what I was planning to do. Zaphodheads generate seperat seats.
I just want to clone two monitors of a 2-screen wide desktop.
The lines:
Option "metamodes" "DP-3: 1280x1024 +0+0"
Option "metamodes" "DP-1: 1280x1024 +0+0"
were changed to to get the signals on the outputs DP-5 and DP-7 as well.
Option “metamodes” “DP-3: 1280x1024 +0+0, DP-5: 1280x1024 +0+0”
Option “metamodes” “DP-1: 1280x1024 +0+0, DP-7: 1280x1024 +0+0”