What you are probably going to need to do is to go into MyVMware, divide the current key into two keys for each pair of hosts. Don't forget that the old key will be invalid when you divide it and it needs to be removed and replaced with the two new keys. Then use the License sets feature in the Usage Meter.
Then again if this is a for a tracking the costs to the users of the system then you may want to consider using Chargeback rather than the vCloud Usage Meter.