You can drag and drop. Click and hold on the far left side of the dropdown you want to move and drag it next to the other, if you want the one you are dragging on the left keep the mouse on the left of the page, if you want to drop it to the right of the other field drag the mouse along the right side of the page. There is a blue insertion marker showing where the field will be placed.
You can also create a second Drop Down field to the right of an existing one either by clicking the plus sign on the right side of a field that is selected, or with the right click context menu options "Insert item beside".
All sorted now.