As long as you don´t take megadoses of any of the two it´s safe. I do believe that ayahuasca has, like aspirin, slight anticoagulant properties. So mega doses could be a bit risky in combination with aspirin taken at the same time. But since you´re ayahuasca journey is going to take place tomorrow and since aspirin doesn´t work that long, it´s not a problem at all.