Placement determines which zones the parts appear in, and where they are placed in that zone.
The zones are configured in Content.cshtml.
You can't combine two separate parts like that using placement. The are two different ways you could approach this:
1.) Create a new zone in an override of Content.cshtml; style members of that zone with float:left so the parts stack horizontally
2.) Create a new shape that combines properties of the two parts. You can do this by writing your own driver, and then create a new view model to pass in as the shape's model