In CSS you have currently only the -xpp-page value in the display property to end a page.
But in a 2 column job, in most cases (if not in all cases) that will not give you the desired result as it will leave the columns on the current page unbalanced.
It is much more likely that you want the equivalent of an ep;bal or ep;aln macro.
Now your only escape route is to create a separate ::before rule in which you output the appropriate macro in the content property.
Things would be a lot easier, straightforward and more CSS like if you could just specify that you want: