If you want only the first and last name fields to appear in a two column format and the remaining to appear in one column, you can adjust the field width for the remaining fields. This will make it appear as if they are on one column.
Check the width of your labels in the form designer.
Oherwise, this is a CSS issue.
You can drag and drop the fields into position via the form editor and then adjust the lengths as needed.
When you drag and drop to re-position the field(s), you'll see arrows around the field indicating the field can be re-positioned to the left or right of other fields (as well as above or below).