How do I produce an index with more than one column?

This produces one column only:
	\startchapter[title=\mytitle{Register}]
	\startcolumns[n=2]
		\placeregister[index]
	\stopcolumns

Nor did \placeregister[index][n=2] or \placeindex[n=2], with or without the columns envirenment, produce the result wanted.
The \startsimplecolumns[n=2] environment did produce two columns, but then played havoc with the textheight.

dr. Hans van der Meer