Redundancy Between two Data Centers

The F5 default recommendation is that you should have an HA pair in each DC, and use F5 DNS (GSLB) to shift traffic between data-centers.

You can sync four devices in a device group, but you can't really build two HA pairs in that group - you would move the IP addresses from DC1 to DC2, which may require some really complex dynamic routing changes on failover.

Basically, the design should accept that an entire DC could go dark - no incoming link, no DC-DC link. The standby site should just function independently, get the traffic, and work.