Consider this: you can use a Y-Link (a Y-coupler connected to a pair of IM 153-2 interface modules) on a S7-400H system, in order to maintain redundancy between the CPUs and the Profibus-DP remote I/O.
However, you cannot use a Y-Link when the redundancy is done through Software redundancy, even if the 2 redundant CPUs are S7-400 PLCs, or if they are S7-300 PLCs. Only the Siemens H-CPUs have the adequate software/hardware combination to support a Y-Link.
Now, do you think your ABB redundant system could possibly have the adequate software/hardware combination to handle I/O redundancy over a Y-Link, if non-redundant Siemens PLCs cannot?
I am afraid you are going to have to look for a solution based on ABB hardware. Maybe someone with more experience with these can give you pointers, unfortunately I do not have any knowledge of any such.
Hope this helps,