RadToolBar does not currently provide the functionality to autowrap buttons to a second row. You'd have to use additional instances of RadToolBars in this case.
Also, be sure that you're setting your toolbar's width using the "style" property:
<telerik:RadToolBar ID="RadToolBar1" runat="server" style="width:100%" Skin="Hay">
I would suggest that you reference these forum posts for some insights in the best approach for handling your requirements:
radtoolbar with two lines of buttons
Hope this helps!