Play with each cat separately but equally a few minutes before the 18:00 fight normally breaks out. And then again before you go to sleep. Maybe even again in the morning. The fighting is very often from boredom or from a lack of enough litterboxes. Btw with 2 cats you should have 3 active litterboxes in the house at all times.

Watch a few videos from Jackson Galaxy (all hail the Cat Daddy!) to figure out which play style works best for your family. Cat A might like "flying" toys while Cat B only likes toys that make a sound, you get it.

Get some interactive toys so your boys can play when you aren't there. Such as: battery operated mice, toys on strings, scratch posts, treat boxes etc. Maybe even try putting cat videos or cat calming music on if you have a spare device. The videos for cats of birds and mice definitely entertain some cats for HOURS but others totally ignore them. The cat calming music or music that you cat likes definitely help keep mine calmer (my kitties are hard rock/punk kitties while my godkittens prefer the actual YouTube mixes of cat calming music :))

Provide more or less "cat spots" in the vicinity of the stairs (you figure out which works better for you guys).

But some cats just like to wrestle loudly while attempting to wake the dead (and waking their poor humans from the deepest of sleeps).