Part of the quarrels run. Verse 20 said it by subtraction; this says it by addition.
Quick answer
Some people are fuel. The verse says a contentious man does to a dispute what wood does to a fire.
Read both lines
Two materials, and they are not the same. Charcoal added to coals that are already burning makes them hotter; wood added to fire makes it bigger. So the first line covers both cases, intensifying an existing conflict and starting a new one, and the second line assigns both to one man.
Read beside verse 20 the pair is complete: remove the whisperer and the fire dies, add the contentious man and it grows. Neither verse says he is arguing with you. He is supplying the conditions, which is why he is often not visible as a participant. The received text carries a scribal variant in the word for contentions, the same one that appears at 21:19 and 25:24, with no change in sense.
Related: Proverbs 26:20 · Proverbs 15:18 · Proverbs 29:22 · Proverbs 30:33
New King James Version®, Copyright © 1982, Thomas Nelson. All rights reserved. Full copyright information. Scripture provided by API.Bible.
Go Deeper
If this verse has been used to put weight on you that it was never carrying, ask about it directly.