Hopper not splitting items between chests.

I'm building a simple automatic furnace. I used to be able to do this by using a double chest and two hoppers to load coal into the back of two furnaces, and the same setup to load my smeltables into the tops of the furnace. The chests would automatically split my items perfectly between the two furnaces, and I'd smelt at double speed. Now, I am attempting to do the same in 1.17.1 for the first time, but when I place my smeltables in a double chest with two hoppers beneath it, only one of the hoppers is used at a time, meaning that only one of my furnaces runs. Is this a bug? Does anyone know a workaround?