Probably identical to: https://ko.infragistics.com/community/forums/f/ultimate-ui-for-windows-forms/97187/ultratabbedmdimanager-is-it-possible-to-have-more-than-one-tab-rows-similiar-to-tabgridtiles
Have a setting in UltraTabbedMdiManager, perhaps in particular in MdiTabGroupSettings, allowing tab overflow to result in multiple rows of tabs.