[tilly]

I agree that a given A/B test should change only one metric at a time. However I've had excellent results from running multiple A/B tests in parallel. As long as inclusion in each is independent and random, the results of each are informative, and if you're concerned about interaction effects you can analyze for signs of a potential interaction in a post-mortem, then do a more expensive multi-variate test if you have cause for concern.