you only need one to operate but better get the 2 running, server will be happy about it and you will not use more power unless you get in Blade system where you have from 2 to 4 PWR sup and power domain.
esxi is fairly simple, you register on VMware and download a free copy of vsphere / esxi. if your server is brand name IE: Dell it should be working out of the box unless it pretty old.
Once installed assign a ip address, mask and GW.
once you have the thing on the network use your windows desktop and go the
https://ip_of_your_esxi
Install the client as web management will stop working once your trial end.
once the VMware vsphere client is installed point it to your esxi box and start managing the thing.
When you download the installation ISO note the serial number you receive. It not really required but it will keep the hypervisor from complaining about expired feature.
one rule of thumb, the total of ram of all your vm on the host must never exceed to the physical ram. You should also keep 1 gig ram for the hypervisor.
Also always assign the number of core you need. If it was not changed in 5.5 hypervisor will wait to have the number of core available before giving cpu time to the VM therefore if you have 4 cores CPU and 2 VM with 2 cores each both of them will receive cpu time together but if you have one 3 cores and a 2 cores they will need to go one after a other. Unless you run all at 100% you should not notice
quick tip, never forget to checkmark connect at startup on your CD rom inside your virtual machine because it will never boot on the cd or ISO for the VM OS install
Last tip always install the VM tools in your VM, will help with pointer and also control of the VM from the hypervisor.
