One thing I have just discovered; the NICs on both my ESXi hosts have the exact same MAC address. Is this a problem?!
That's definitely an issue and most likely the reason for what you see!
In case this is a virtual lab, change the MAC address of one of the hosts. In case you cloned the virtual ESXi hosts you may also run into more issues when you try to add them to a vCenter Server environment!?
André