First, your crossover cable does not need an IP. LLT (Low Latency Transport) does not use IP. It does not need to be plumbed up for LLT to work.
If you don't have a SAN, you need an external disk drive, such as a disk array, attached to both systems via SCSI cable to SCSI controllers. Then both systems can see the external disks.
Using Veritas Volume Manager, you create disk groups that can be imported and deported from the systems.
With Disk Groups, you can implement IO Fencing thru VCS, it is actually the Disk Group agent that applies the SCSI3 reservations. Your disk array will have to support SCSI 3 Persistent Reservations (SCSI3-PR) for this to work.