This patch changes the default thresholds that the ePub reader uses to decide how many columns to display on a page. It doesn't affect the KePub reader.
Paste into librmsdk.so.1.0.0.yaml:
If you reduce these threshold values then the reader should switch to multi-column mode at a larger font size setting.
If you Increase these threshold values then a smaller font size setting should be required before the number of columns increases. If the thresholds are made very large then only a single column should be used for all font size settings.
Note that the patch just sets the default thresholds, the publisher can still override them in the book's XPGT stylesheet. You might want to use the "Modify ePub" Calibre plugin to remove the xpgt stylesheets from books before sideloading them.
(Also note that the font-size threshold is based on the size set by the device font-size slider, which is not necessarily the font size used for paragraph text since the publisher might have changed the paragraph font-size in the book's CSS stylesheet.)
I haven't tested this much, just a little on my Glo with firmware 4.8.11073, but it should work the same on any firmware version.