According to Ibuprofen Could Be Bad for Heart Patients, ibuprofen blocks aspirin from thinning the blood:
Fresh evidence adds to suspicions that ibuprofen could be dangerous for most heart patients because it can block the blood-thinning benefits of aspirin.New research published this week in The Lancet medical journal found that those taking both aspirin and ibuprofen were twice as likely to die during the study period as those who were taking aspirin alone or with other types of common pain relievers.
Scientists believe ibuprofen clogs a channel inside a clotting protein that aspirin acts on. Aspirin gets stuck behind the ibuprofen and cannot get to where it is supposed to go to thin the blood.