Currently, autoland won't land attachments from other bugs. It seems to specifically check that all the attachments you specify correspond to the current bug. The thing is, sometimes the patch from one bug will only apply on top of the patch from another bug. In that case, right now you either have to fix the merge conflict yourself and submit the patch manually to try, or wait for the first patch to make it to mozilla-central so there's no merge conflict. It would be nice if it applied all the patches as long as at least one is from the current bug, or if it just didn't check at all. Motivation in my case: bug 719515 comment 6, bug 719523 comment 2.
This is not a current goal of the Autoland project and is incredibly complex to implement. I'm going to resolve this as wontfix and at some point down the line when Autoland is actually product-level and running with the bugzilla extension instead of whiteboard tags we can look into the possibility of linking patches from other bugs through the extension.
I meant 'production-level' as it is currently very experimental.