Reference 1 refers to studies that trail various combinations of analgesics. Of note is the study concerning paracetamol (acetaminophen) and ibuprofen.
The study compared the adverse effects of 4 groups:
- paracetamol and ibuprofen, and;
They found "adverse events (signiﬁcant difference)" consisted of:
- Vomiting: 24%–32%
- Drowsiness: 5%
- Abdominal pain: 3%–10%
And stated that the "paracetamol group was drowsier than other groups."
Reference 1 states: "Combining [the two] analgesics may increase the incidence of adverse effects."
A similar clinical trial was described in Reference 2. The paper found that adverse events were "numerically higher in the groups receiving combination tablets."
From this it can concluded that the likelihood of experiencing adverse effects is increased by combination of the two analgesics.
I could not find anything solid in regards to your last question. The increased likelihood of experiencing adverse effects may be due to the two agent's similar mechanism of action. Reference 3 states: "Due to their mechanisms of action, using paracetamol and ibuprofen together theoretically increases the risk of renal and hepatic toxicity." It is possible that this extends to other adverse effects.
- Combining Paracetamol (Acetaminophen) with Nonsteroidal Antiinﬂammatory Drugs: A Qualitative Systematic Review of Analgesic Efﬁcacy for Acute Postoperative Pain, http://www.endoexperience.com/documents/APAPOngetal.pdf
- Paracetamol, ibuprofen, or a combination of both drugs against knee pain: an excellent new randomised clinical trial answers old questions and suggests new therapeutic recommendations, http://ard.bmj.com/content/70/9/1521.full
- Evidence that alternate dosing of paracetamol and ibuprofen in children with fever may reduce temperature: other benefits uncertain http://www.bpac.org.nz/BPJ/2014/July/docs/BPJ62-news.pdf