Siemens S7 Redundant system interface using Modbus communication

Hi,
I need some help to communicate the S7-400 redundant system using CP441 to third party devices on Modbus protocol.
We have following configuration -
S7-400 redundant system having 2nos. CP441-2 in each CPU rack.
The first CP441 is networked to Device 1 as PTP1
The second CP441 is networked to Device 2 as PTP2
As this is redundant PLC the 2 CP441 in the standby rack are networked as - Device 1 as PTP3 , Device 2 as PTP4
The logic addresses for lead CP441's are 1000 & 1001
& standby CP441's are 1002 & 1003
Here's the problem -
The Modbus protocol requires a ladder call to individual CP441 modules to be written in OB1. Do I call all the 4 modules seperately or call just 2 modules in the lead processor?

How do the standby CP 441s seamlessly takeover once CPU becomes lead?
I am unable to connect the standby 2 CP441s to PTP1 & PTP2. Also it does not allow me to configure same logic addresses of 1000 & 1001.
Do I assign same slave addresses for the standby CP441s as in lead rack?

Would appreciate any help on this.

Thanks in advance.

Pally