Can I add an ESXi host to more than one vCenter server?

Solution 1:

The host can only be managed by one vCenter at a time. But you can move hosts to new vCenters in this manner without disrupting the virtual machines.

Solution 2:

No, you can't do that. The vCenter management stack assumes exclusive ownership of the ESXi hosts that it manages.

It will warn you when you try to add a host managed by another server, and if you insist and go ahead, the other server will no longer be able to communicate with this host, and will show it as "dead".