How to make event registration form fields be displayed inline
If you are on Drupal you could do the event registration using a webform with webform_civicrm and use the webform_layout module to place two fields on the same line.

Alternatively you could write a tiny extension to drop in some jQuery to move the fields around in the standard event registration form. There is write up of how to do that here: http://jackrabbithanna.com/articles/easy-jquery-modificaiton-civicrm-forms  Note that since that was written cj has been deprecated in favour of CRM.$

If you can do the changes with CSS without needing the power of jQuery, you can also do that - see my answer to How can I change the way price set information is presented on a contribution page?