You can run both ESX host in vCenter with diffrent keys. One for vSphere 6 and one for vSphere 7. When you have an active support agreement, you can upgrade your license keys to vSphere 7 and add the new key to vCenter.
The limitation for vSphere Essentials Kit is, that you cannot have more than three esx hosts (version does not matter) and 6 cpus in one vCenter.
Please mark as resolved, when your question is answered.
Daniel