As a parent, your priority is to do what's in the best interests of your children. You have two children who don't get along well. One is a teenager, and the other is still in elementary school. They're several years apart in age and have completely different personalities.
Your concern is that the older child and younger child aren't happy when they're together, and adding the stress of the divorce and going between homes might heighten that issue. They could become more jealous of the other child's time with each parent, and that's problematic.
Is it possible to separate siblings after divorce?
There is an old saying that you don't choose your family, and that's true even in childhood. Children don't get to choose their siblings, and there is a possibility that it would be better for both children to live apart. For example, if an older sibling is aggressive to a younger sibling and regularly scares or taunts them, separating them for their peace of mind may not be a bad idea. If a younger child needs more attention and you notice the older child becoming jealous, creating custody days where they don't have to share that attention could be helpful.
Separating siblings around the clock probably isn't what's best for them, but it is possible to set up a schedule where they're not together all the time. You could have the two spend time together on weekends but have them in different homes during the school week, for example, so they can focus on their educations and not arguments or negative interactions with each other.
There is a balance in custody schedules that parents should aim to maintain
Trying to work out a custody schedule for siblings who don't get along can be tough, especially because keeping them apart may not help them resolve their differences. Keep in mind that you can set up a split custody schedule that works now and modify it in the future if necessary, allowing you to do what's in your children's best interests now and adapting as their relationships change, for better or worse, in the future.