That's a great point Dory.
Make sure to declare the champion in advance of the next cast, otherwise both the challenger and the champion will go out again with the same split. Note that there are two ways to A/B test an email, with the email itself and with an email send. With A/B tests for an email, the test is split and the entire audience recieves the message. This applies for the traditional email A/B test.
Nick, are you running an A/B test within an email send program that sits in the engagement stream? You mentioned the remaining audience.
For A/B Tests within an email send program (depending on how you set the test up), only a portion of the audience recieves the test. Once a winner is chosen the rest of the audience receives the champion message (this message can be scheduled). After you have chosen the champion message for the email send program in the engagment stream, the champion will be sent on the next cast for the stream. If you do not choose a champion before the next cast, the email will still be in limbo and I do not believe the email will be cast again (until you choose a champion).
Hope this helps!