Hi,
The two columns are potentially due to your template which is overriding some properties.
For example, if it is adding margin on the elements so the width is maybe too high to keep the elements one next to the other one.
So you have potentially to reduce the element's width with a property like:
.hikashop_checkout_login_left_part{width: 30% !important;}
.hikashop_checkout_login_right_part{width: 60% !important;}
To change the default selected value, you have to edit the view "chekout / login" and change the value of the variable "$defaultSelection" to "0".