Presently, this is impossible.
The footnote area has the same layout ad your main text (it is not an independent area; it is borrowed from the main area). Consequently, you can mitigate waste of space by configuring your page style(s) for 2-column. But this may have adverse aesthetic effect on your text (chapter headings, lists, …).
There is also a pending enhancement request triggered by a similar question (which one?) asking for independent layout of footnote area, so that its number of columns can be independent from main text (I don’t remember its number).