I think that you need to use the champion challenger option. The required steps are quite well outlined if you follow the following articles:
https://community.marketo.com/MarketoArticle?id=kA050000000LGX7CAO
This will allow you to apply the A/B criteria to your email. However, as far as I am aware it is up to you to declare the winner, so you couldn't send the 20% test separately every Tuesday and then the winner automatically every Thursday.
The Email program can automatically send the winner but as far as I know you cannot reference an email program in an engagement stream.
@Josh, I'd be interested to hear more what your option is?