As the PP said, normal one-day/one-park tickets can be upgraded. You must do the upgrade before the end of the last day the ticket is used (but you'll obviously want to do the upgrade before that so you can get into DCA). Just go to one of the ticket booths and they can help you.
As the PP said, complimentary tickets and a few other types cannot be upgraded. Tickets that cannot be upgraded usually say so in the fine print on the back.
Also, just to make sure, but you cannot combine multiple tickets to get a single upgraded ticket. I'm fairly sure you mean that two of you are going and each of you have a one-day/one-park ticket, but I just thought I would mention it.