WPF ToolBar: How to arrange toolbars at design-time

I've got two Wpf ToolBars in a single ToolBarTray. How to make them fit on two rows?

I noticed users can move them at run-time. Is there a way to get the same behavior at design-time without using two ToolBarTrays ?

To sumarize, at startup, I want this:

instead of that:

Thanks