From what I've been told by doctors and nurses since high school, is that Ibuprofen is associated with kidney problems whereas Acetaminophen is associated with liver problems.
If you're in a lot of pain and these are all you have available, it is actually better to alternate small amounts of both than to take large amounts of either one.
Acetaminophen: You should never exceed 4000mg of Acetaminophen in a day, but toxicity would only likely occur if you take 4000mg per day consistently. A sudden toxicity could be caused by an intake of about 7000-10000mg in a day in an average person, or at 150mg/kg. I currently weigh around 205, so that would make almost 14000mg! It would take a suicide attempt to reach that level in a day. That's why most occurences of toxicity are due to chronic high intake. The numbers look funny, but that's because the toxicity rate is variable from person to person.
Toxicity could cause liver damage or failure, possibly leading to death if not treated quickly.
Ibuprofen: I've been directed by a doctor to take as much as 1000mg of Ibuprofen at a time up to six times daily for back pain, but that doesn't even get close to the toxic baseline of 100mg/kg. That would be over 9300mg for me. Oh, by the way, this is on a dose by dose basis. I'd either have to take 50 ibu tabs in a day, or take ibuprofen at high doses daily to get toxic effects.
Toxicity usually leads to abdominal pain, dizziness, nausea, headaches (ironically), and all those typical symptoms you always hear about, except for maybe anal leakage. I don't think it causes that. Anyway, more severe effects have been reported like kidney failure, seizures, cardiac arrests, or GI bleeding, but these are the extremely rare cases you always hear about. Kidney failure is the big one with Ibuprofen.
I do not encourage anybody to take either of these unless it's absolutely necessary. I don't even like to take supplements unless I really need them. The super high dosages that lead to toxicity actually shocked me because they make it seem like it would be really hard to suffer any ill effects. However, I am still skeptical of their safety, even in large doses suggested by a doctor. I've expressed this concern with a doctor when I hurt my back a long time ago, and they encouraged me to stagger Ibuprofen and Acetaminophen in smaller doses if I was uncomfortable taking that much ibuprofen. I was never told to take them both at the same time, but there would be no adverse effects from taking both multiple times throughout a day.
Just a tip... take a dose with a glass of milk or something to eat. Consistently taking these will be dangerous for your stomach, and giving it something to sit with will be much safer.
I guess my summary would be to forget about the dosages that may lead to toxicity, and only take what you think is absolutely necessary. While both Ibuprofen and Acetaminophen products are very safe, they can easily be abused. Never take either one consistently for weeks at a time. If you have chronic pain, see a doctor, don't rely on pain relievers to get you through life. I hope this helped someone.