Should I use acetaminophen and ibuprofen together in a child to reduce fever?

You can - but why? Fever is a generally beneficial response to infection and helps to fight the infection. The only reasons to reduce fever are that the individual is patently uncomfortable, or that their heart or lungs cannot tolerate it, or that the fever has reached heights in a child which place them at risk of seizures (>105). You can use tepid washcloth baths, low dose pediatric tylenol, (acetaminophen) etc., .
Yes. Fever is your body's response to foreign invaders whether bacterial or viral. A good high fever makes it harder for the infection to grow and spread in your child's body. If your child is uncomfortable or in pain, then giving acetaminophen or ibuprofen, is reasonable. If your child is otherwise happy, encourage fluids to reduce any dehydration and leave the fever to do its work.