Licence for backup webserver

Do we need separate licences if we have 2 servers, one idling but acting as backup for the other (but both “live” as in accessible from the Internet)

If you are willing to run Pro versions on both, then the answer is – yes, you would need two separate licenses, as a license is only meant for one server. However, in your case you could also use GPL version on one and Pro on the other, which should work just well, in your case.