What this means in practice
Taking the two together is reasonable for short-term pain, and the doses that work are lower than the maximum of either. The thing that actually has to be tracked is the acetaminophen total across everything taken that day, including anything for a cold, a headache or sleep, because that is the number with a ceiling. Nothing here requires buying a combined product; the same effect comes from the two taken separately, which has the advantage of making both doses visible.
What people say
"Take them together, they work on different things." Common informal advice, and equally common is its opposite, that doubling up on painkillers is reckless. Both circulate without anyone naming what the combination actually buys.
What the evidence supports
In a synthesis of 39 reviews covering roughly 460 trials and 50,000 people with pain after surgery, the best-performing option of 53 reliably-evidenced drug and dose pairs was ibuprofen 200 mg with paracetamol 500 mg: six or seven people had to take it for one more of them to get at least half their pain relieved, against about two and a half for ibuprofen 400 mg on its own. Combining ibuprofen 400 mg with paracetamol 1,000 mg was also among the longest-lasting options, working eight hours or more. The authors are explicit that no statistical comparison between drugs was made, so this is a set of separately measured results placed side by side rather than a proven ranking.
How it is supposed to work
The two work through different routes, which is the proposed reason combining them adds up. Anti-inflammatory painkillers act mainly on inflammation at the site of injury; acetaminophen has little anti-inflammatory action and is thought to act largely within the nervous system. Neither competes for the other's target, and their dose-limiting harms fall on different organs.
Which way it runs
Mutual rather than one-way: each improves on what the other achieves alone, at doses lower than either would need by itself. Demonstrated for short-term pain after surgery, which is the setting these measurements come from.
What is still missing
The measured advantage is real but it is an advantage in short-term pain after surgery, which is the model almost all of this evidence comes from, and the same additive logic that makes the combination effective is what makes accidental double-dosing easy when the acetaminophen arrives inside a cold remedy nobody counted.