In order to run a successful A/B test, it is crucial to keep these two points in mind:
1- Test one metric at a time : The most important aspect about a A/B test is that, one should only test one metric at a time. This means that when running an A/B test, all other settings of the test campaigns need to be exactly the same.
2- For Marvin optimized campaigns, make sure to have min 10 conversions per day. Budgets need to be high enough so that each ad set in the test campaigns can buy a minimum of 10 conversions per day. Thus, extra budget might be needed for a statistically reliable A/B test.