Hi Elicia, from what I've seen, you cannot force an email program to send the winner in less than a 4 hour window. Another option would be to use the champion/challenger test feature to send out a percentage of your emails, and then send the winner to anyone who didn't receive the first email. Because this requires both manual review (to select the winner), and a lot more setup since champion/challenger emails can only be done as triggered emails, I wouldn't suggest this option.
Would it be possible to send as a single batch and use your findings to improve your subject lines week over week?