Double after split?

Been playing blackjack on the ferry tables for years and I’m still not totally clear on what’s best after you split. When you split a pair, are you supposed to double down if you get something like an 8 or 9 on one of the new hands, or just hit and play it safe? I’ve heard different takes depending on who you ask. Some folks act like doubling after split is a no-brainer if the dealer shows a weak card, but then I’ve lost a chunk doing it when my luck turned.

The online tables make it even weirder since not all of them let you double after a split. I mostly play at tables with four or six decks, if that matters for the math. Anybody here actually track their wins and losses on hands they doubled after a split? Has it really paid off for you or is it one of those moves that sounds smarter than it is? Curious what everyone’s real-life experience has been.