Toolbars cannot be stacked vertically so that they are stacked on top of each other on either side of the ArcMap Table of Contents.
This is a known issue.
- Start Windows Explorer and navigate to: C:\Documents and Settings\<your_login_folder>\Application Data\ESRI\ArcMap\Templates.
- Delete or rename Normal.mxt.
- Start ArcMap.
- Move and stack the toolbars as desired.
- Save a blank map and exit ArcMap to save the changes to the new Normal.mxt.
If it is necessary to move the toolbars again, repeat the steps above.