Hi Vitor,
This is a good use-case for multiple repeatable zones in a slice. The best workaround I can think of (and I know it’s not great) is to create a custom type for the side content, then use a Content Relationship field in the non-repeatable zone of the slice to select the side content for that slice.
The team are aware of this feature request and hopefully, it's something they can consider in future changes. It's not being worked on right now, if/when this changes the team will update everyone here.
Thanks.