Unfortunately the feature is not supported.
If multicolumn is needed for a single page only, you may play with undocumented
<pd4ml:layoutbox>content</pd4ml:layoutbox>
It "understands" the following attributes (numbers are default values):
maxcols=4
colspacing=20
scalelimit=0.75
scalestep=0.02
width
height
The purpose of the tag is to try to fit given nested content into a box, limited by height/width. It scales the content down till
scalelimit. If a single column is not sufficient for the content, it tries to layout into 2 columns etc.