You have two sites, X and Y. You have two switches at each site, A and B. Four switches: X-A, X-B, Y-A, Y-B.
Switches X-A and Y-A have one or more inter-switch links (ISL). Switches Y-A and Y-B have one or more ISL. There is a maximum round-trip limit of 5 milliseconds for a switch message to go from one site to the other and back.
There is NOconnection from any A switch to any B switch.
One 8400 is connected to local switches X-A and X-B. The other is connected to local switches Y-A and Y-B. HPE 3PAR Remote Copy is then configured between the two 8400 systems.
For redundancy, a host would need two HBA ports, with physical connections to the local A switch and the local B switch.
I recommend you contact Hewlett Packard Enterprise for in-depth analysis and consulting on your environment and to provide assistance with getting Peer Persistence set up between your two sites.
Best Regards