Hi,
I'm presuming you're using a version that has the CUI-based customization environment. When you have the Custom Toolbar active in the editor, look in the Properties -portion of the editor.(On the right) The Rows-value will give you an option to make the toolbar to split into as many rows as you want (or columns as the case may be) The Orientation in turn lets you change the Vertcal/Horizontal aspect.
HTH
Petri