If Trump and Biden both died today, what would happen?
Imagine the following scenario: On the 9th of January 2024 in the morning we wake up to news that President of the USA Joe Biden suffered a stroke and died in White House. The administration confirms that the death was entirely natural and no third party contributed to it. Even before the news reach all TV stations another one breaks: Former president of the USA Donald Trump has suffered a cardiac arrest and died in his residence in Mar a Lago. Early reports claim that the site was secure and no malicious actor could reasonably reach Mr. Trump so the death is most likely natural.
Neither have been elected yet. The Democrat and Republican parties would both nominate new candidates for president and then people would vote on them.
If you're talking about after they've been elected then the vice president takes over as president, followed by the speaker of the house if the vice president is incapacitated.
Important to note that the speaker only takes over if the president and vp died together. If the president dies, the vp takes over and picks a new vp, who would then be first in the line of succession.