I know this is a lot more difficult than it sounds because attention is a finite resource, but giving each of them the attention they need. This does not have to be the same amount for each child but none of them should feel like the other one is getting attention when they need it.
Most adults I know that don't get along with their siblings felt like they were neglected compared to the other sibling(s). Whether that's objectively true is hard to tell but it's worth checking in with your children from time to time.
Generalising statements suck in general.
Relevant xkcd: