Hi,
Currently it is possible to open second pane on the right or on the bottom, but those panes are used only for previewing. I never use this previewing feature, but using other editors I often split editing area horizontally or verticaly in order to edit the same document in two places at the same time.
It would be nice to have such use for the right or bottom pane in WeBuilder; good examples : Komodo editor from http://www.activestate.com and PSPad editor from http://www.pspa.com.