While it is a bit dirty you can have a carousel widget with 2 cells, each hosts the treetable.
TreeTable in the first cell doesn’t have the treeitem template in column’s config. So you can’t open branches but can select the top level items only. The second cell of carousel hosts treetable which is bound to the component in the first cell and shows a list of item in the selected branch.
https://docs.webix.com/desktop__binding_details.html#bindingrules
In such case, after selecting item in the first table, you can just show a second cell, where will be a table with the list of related items.