For EASA-land operators, the regulation is CAT.OP.MPA.180
(c) The operator shall select two destination alternate aerodromes when:
(1) the appropriate weather reports and/or forecasts for the destination aerodrome indicate that during a period commencing one hour before and ending one hour after the estimated time of arrival, the weather conditions will be below the applicable planning minima; or
(2) no meteorological information is available.
Pretty straightforward. It is a while since 've operated to FARs, but I am sure it is the same somewhere.