Maybe I'm just being pessimistic
While I can see that one SAP system could support a load of local councils (not just 2), so provide good value, the public sector IT record says we will probably be reading "lessons learned" in about 2016, once this is all scrapped..
It's not even like Haringey has a good record so far...
http://www.lynnefeatherstone.org/issues/haringey-council