Their prices tend to rise and fall according to the same financial/political forces. They're both seen as real money by a tiny (very wise) fraction of the population and as atavistic relics by the vast, ignorant majority. And – most important – they will both preserve their owners' purchasing power when today's fiat currencies evaporate like the fever dreams they always were.
So you definitely want some (and maybe a lot) of each. But gold and silver are not identical. They have different strengths and weaknesses in various "monetary reset" scenarios. And their prices don't move in lockstep. Sometimes one is cheap relative to the other.