You can install vCenter as either a virtual machine (either the appliance or the Windows based version) on any ESXi host you like or even on a physical Windows host. It communicates with the ESXi hosts over the network. The recommendation however is to install vCenter Server as a virtual machine.
Regarding the two vCenter Servers. Why do you want/need two vCenter Servers? Depending of what you are using vCenter for, there's usually almost no impact if vCenter Server is down. Besides that, vCenter Server cannot be installed on a domain controller.
André