Failback Without Second Core

My team is trying to test failover of production systems for a customer. They have five Windows server VMs on an ESXi host that each have a virtual standby on another ESXi host, but only one Core. There is a SQL server and an Exchange server involved. My question is, after we complete failover and testing, what would be the best way to failback those VMs? What we have in mind so far is exporting the VMDK of the virtual standby and recovering the production VMs. The issue is that any time in between is going to result in data loss. If anyone knows a better way to minimize downtime, it would be greatly appreciated. They want to test during working hours and we are still working out how much data loss is acceptable to them.