Authorship order in life sciences is intricate and has many problems. By convention, order is by the amount of contribution (decreasing). Also by convention, the PI/supervisor/"senior author" appears last. Things start getting complicated when you have many contributors.
I would like to focus on the end of the author list. When there are several PIs, they all appear at the end. However, if you consider the before-last author, a reader would not be able to know whether this is the author with the least contribution or alternatively a co-PI. I have seen different "solutions" to this problem (e.g. marking all senior authors as corresponding authors).
I would like to know what kind of solutions you have seen for this problem, and whether these solutions are recognized by institutions and funding agencies.
For those of you wondering why these intricacies matter - authorship order can play an important role in getting a faculty position, awards, grants and tenure. For example, for some fellowships you may only list publications for which you are the first author.