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.
So how much of each should we own now, and how quickly should we plan to load up the truck? The answer is different for each person, but a few things are generally true.
The gold/silver ratio
The relative prices of gold and silver tend to fluctuate within a broad but discernable range. This gold/silver ratio is expressed as the number of ounces of silver it takes to buy an ounce of gold and tends to rise and fall along with the emotional state of precious metals investors. When those investors don't foresee imminent inflation or other monetary disruptions, they gravitate towards gold's safety and stability, and shy away from silver's volatility. Gold's price rises relative to silver's, producing a high gold/silver ratio.
When investors expect rising inflation or other kinds of currency instability, they buy precious metals generally, but gravitate towards silver's greater upside potential. Gold and silver both rise but the gold/silver ratio falls as buyers push silver's price up more quickly than gold's.
These fluctuations typically happen within a range of 40 to 80 (i.e., 40 to 80 silver ounces per ounce of gold), with a high number implying that silver is cheap relative to gold and a low number meaning that gold is cheap relative to silver. Breakouts beyond this range in either direction are useful signals.