It checks the two that are pushed to your desktop. I do not know of a way to check them individually or to pick which one. I do not have two set for my phone, but it is my understanding that diagnostics you just run and let it do what it has to do.
You can try going into OPTIONS> ADVANCED OPTIONS> MESSAGE SERVICES
That might let you pick one or both of them to push to your mobile. Then pick the one that you wish to diagnose.
I only have one email pushed to my phone, so I do not know if this will work.
Life is like a movie, you've sat through half of it and its sucked so far, it probably isn't gonna get great right at the end to make it all worthwhile. you shouldn't get blamed for walking out early