I know it's confusing, but the ConnectedMonitor and CustomEDID options apply to GPUs, not X screens, but xorg.conf has no syntax for specifying GPU options. Your best bet is to specify the same settings on both screens on a single GPU. E.g., for screen2 and screen3 (which share the GPU "PCI:4:0:0"), set it like this:
Option "CustomEDID" "DFP-1:/root/SyncMasterEDID2.bin; DFP-2:/root/SyncMasterEDID3.bin"
You shouldn't need ConnectedMonitor, so try deleting it first, but if that doesn't work then use this:
Option "ConnectedMonitor" "DFP-1, DFP-2"