Did the combination of the two VCenter license keys work to use only 1 key on a single VCenter server?
Yes and No because your write this questions but i think your intention was something different i think. Getting one Key which contains several Instances of vCenter is possible because IIRC my NFRs showing that for our vCenter Std. But with that you doesnt overcome the limitations of the edition.
1. vCenter Essentials = Max. 3 Hosts ESXi Essentials
2. vCenter Foundation = Max. 4 Hosts ESXi Std/E+
3. vCenter Standard = Unlimited Hosts ESXi Std/E+
As you see the differential in the Edition are "only" the number of managed hosts rather than "features". Its not possible to work around this limitation rather than upgrade to the right edition.
Regards,
Joerg