After some digging around, I found a hackish solution by modifying the notebook css.
This works with IPython 2.0, but may not work with 1.x!
To test this approach simply execute the following in your notebook
%%HTML
<style>
div#notebook-container.container {
/* This acts as a spacer between cells, that is outside the border */
margin: 2px 0px 2px 0px;
list-style: none;
padding: 0;
margin: 0;
-ms-box-orient: horizontal;
display: -webkit-box;
display: -moz-box;
display: -ms-flexbox;
display: -moz-flex;
display: -webkit-flex;
display: flex;
justify-content: space-around;
-webkit-flex-wrap: wrap;
flex-wrap: wrap;
-webkit-flex-direction: row;
flex-direction: row;
}
div.cell {
width:550px
}
</style>
This way you get a flexible box layout and thus you can have two cell floating side by side.
As I'm not an CSS expert this is for sure a rather weak hack, but it seems to work reasonable. To use this approach more seriously you might create a new profile and add the css to your custom.css.
I found some inspiration here.
The result looks like this
