I'd like to have a two column TOC but at present InDesign doesn't seem to recognize the width of my columns. I tried adjusting the tabs and recreating the TOC from scratch but it had no effect. The type styles are set up to be justified left. I created these "columns" by making two 2.0625" wide text boxes and threading them together. CS4 on a Mac.
Use a right indent tab.
As Bob says, a right indent tab will force the tab to the edge of the column.
You insert it by opening up the More Options in the TOC dialog box, and then use the Between Entry and Number field to enter the code for right indent tab. Or use the menu to choose it.
You may have to fiddle with the tab settings for the entry style in order to get the tab leaders to show correctly.