[DesignerSerializationVisibility(DesignerSerializationVisibility.Content)] public IList Layouts {get;}
[DesignerSerializationVisibility(DesignerSerializationVisibility.Content)] public IList Layouts {get;}
Because this is a "Content" property, one can initialize an instance of MultiLayout in XAML just by creating nested elements that are themselves layouts.
Nesting of MultiLayouts is not supported, since it would probably not be useful.