Сачин: Поместите свои кнопки ленты внутри RibbonGroup.
<my:RibbonTab Header="Home">
<my:RibbonGroup Header="File">
<ribbon:RibbonSplitButton
Label="Open"
LargeImageSource="/Images/Folder Open.png"
Command="{Binding OpenFolderCommand}">
</ribbon:RibbonSplitButton>
</my:RibbonGroup>
</my:RibbonTab>