If I use a SplitButton inside of a Menu when creating my xml to load in to the ASP.NET RibbonBar, it does not place the SplitButton buttons in any kind of dropdown. It places them at the same level of Menu. Here is the XML:
<?xml version="1.0"?>
<RibbonBar>
<ApplicationMenu Text="Home">
<ApplicationMenuItem Text="Item1" />
</ApplicationMenu>
<Tab Text="Tab1" Size="Small" ImageUrl="/Images/Image1.png">
<Group Text="Group1">
<Button Text="Button1" Size="Large" ImageUrl="/Images/Image2.png" />
<Menu Text="A Menu" Size="Medium" ImageUrl="/Images/Image3.png">
<MenuItem Text="MenuItem1" Size="Large" ImageUrl="/Images/Image4.png"></MenuItem>
<SplitButton Text="SplitButton" Size="Large" ImageUrl="/Images/Image5.png" ImageUrlLarge="/Images/Image5.png">
<Button Text="SplitButton_Button1" Size="Medium" ImageUrl="/Images/Image6.png" />
</SplitButton>
</Menu>
</Group>
</Tab>
</RibbonBar>