AC800F and Rack IO
with regards to hardware I would use AC 900F today. It is even more powerfull than AC 800F, but also supports only one CI910 per CPU.
The limitation of an FI810 or CI910 is the same like with the DCP10. It only supports 5 racks (5 x 9 I/O modules). If your two rack I/O nodes combined fit into those 5 racks, then you can merge them into one node.
But note that the I/O cycle times change by adding more racks. Each rack adds 10 ms cycle time for analog signals and 2 ms for binary I/Os.
If for example you currently have one node (node 1) with 2 racks and one node (node 2) with 3 racks, analog signals on node 1 are sampled with 20 ms cycle and binary signals are sampled with 4 ms. For node 2 the numbers are 30 ms and 6 ms respectively.
If you combine them into a single node with 5 racks you get 50ms for analog signals and 10 ms for binary signals.