There is a 'Add Layout' tool that can split an area into columns. It's flakey at times so don't go crazy with it. (I can't believe I'm recommending it because it has caused many people a lot of trouble)

I've also had some luck using the 'Design' option on the block/stack and add 'max-width:300px;float:left' to the CSS

The best solution is to create a new Page Type file that has DIVs arranged into the number of columns you want with an editable area in each DIV but that requires a fairly decent command of the HTML/CSS in the underlying Page Type files in your theme folder.