I'd be very interested in tracking multiple metrics against an experiment too. Has anyone used Vanity to do this?
Currently, Vanity lets you specify multiple metrics in the experiment definition, but it lumps them all together when tracking conversions (a user triggering any one of the given metrics counts as a converted user). I looked into the code a little and it seems we'd need to change how some of the data is stored to make this possible.
This article has a great example of what I have in mind:
http://bjk5.com/post/28269263789/lessons-learned-a-b-testing-with-gae-bingo. Sadly, his tool is actually a Python port of A/Bingo.
Ian