We had recently been using a "pipeline splitter" smart campaign to A/B test an activity-based smart campaign pipeline of emails versus a time-based pipeline. I found out from a Marketo support rep that the logic we had been using to drop 50% of new leads into campaign A and the remaining 50% into campaign B would not work correctly because it was designed for batch campaigns rather than triggered campaigns.
She suggested I try the Champion/Challenger A/B tester because it worked with triggered campaigns. However, I soon realized this didn't really solve my problem since the champion/challenger a/b tester could only test a single emails as opposed to entire smart campaigns.
So the only way we can really test an activity-based versus time-based campaign is to have 100% of new leads go into an activity-based campaign for a set period of time, and then switch to 100% of new leads in a time-based campaign. It's disappointing that Marketo is unable to A/B test two smart campaigns from a triggered campaign source simultaneously.