I don't know exactly how you planned to allocate addresses to your redundant scopes, but it's something to look out for. Another thing to be aware of is the way clients on different logical subnets on the same physical subnet communicate in such a situation. There are basically two ways. Firstly you allocate a default gateway in each of the scopes that corresponds to the primary or secondary address on the Cisco for that subnet. This will override any default gateway that is assigned in the DHCP options.
All clients then assume that they are all on the same subnet and ARP for every address.