Try this...

This form has 5 fields, and 2 custom HTML elements. 

The custom HTML elements are between question 3 and 4 and after question 5.  These are empty fields, they are just there to separate the formatting between the sections and make it clearer.

Questions 1 and 4 have the CSS Class of leftSide.

Questions 2, 3, and 5 have the CSS Class of rightSide.

The fields are sized to "Extra Large" which fills the full width of the span that is around the box.  The CSS below sets the width of the span to 50%, so if the fields are set to Medium for example, they'll only take up 25% of the width of the form (50% of the span which is of 50% of the form).

It looks like this on the Layout page:

Here's the CSS code: 

.leftSide {float : left; width : 50%;}
.rightSide {float : right; width : 50%;}

And then here is the form preview:

Hope this helps!