due to the fact that I have added own entries to the palette, the palette is getting too large and not all entries are visible on a small screen.
How can I extend the width of the palette so that it has two columns instead of one?
this is a styling issue, which can be solved editing the CSS file in diagram-js. The width of the palette is defined by the width of its content. You could for example try to set a display: inline-block attribute for all palette entries which should be displayed in parallel. But since this is a pure CSS issue there are many possible solutions and maybe you are better preserved in a CSS forum for that