Is there a way to show second country at top of list of countries (after the default country) in CiviEvent?
Do you have participants from anywhere other than US/Canada? If it's just US/Canada you can restrict the country list to just those two (or some handful) under Admin - Localization - Languages, near the bottom of the page.

If you really do want all 200+ countries in the list, you can do a hack like this if you have access to mysql:

update civicrm_country set name=' Canada' where name='Canada';

(note the blank space in front - this will make it float to the top)

This might get overwritten on a future upgrade if Canada changes its name, but that seems unlikely. Still, this isn't a recommended hack, just if you're desperate.