Can I take 400mg of ibuprofen and 500mg of acetaminophen at the same time? And how often can it be done?

Yes. You can take them together, however u may get more benefit alternating them - for example 4-6 hrs separation. However do not take more than 4000mg of Acetaminophen in any 24hr period. You can do this for a couple of days - any longer pls see a doc.
Safe combo. Ibuprofen 2 pills every 6 hours and Acetaminophen 2 pills every 6 hours. This is your maximum 24h dose. Rec.: talk to your physician for more information. Hope it helps.