How to shorten the list of authors in case of joint first authorship?
For things like the list of authors at the top of a page, the authors don't put those into their own manuscript at all: they are added in by the publisher during layout. The publisher will work to their own style guide. That style guide may be based on some combination of tradition, design aesthetics and user feedback. 

In those rare cases where the author is responsible for putting these in, here are the rules, in descending order of precedence.


Do what the style guide says.
If that's not clear, ask the editor (for a journal paper) or your supervisor (for a thesis)
If you still don't have a clear answer, do what other recent documents that are most similar to yours have done (so for theses, this would be the most recent theses in your department)
If you still don't have a resolution, use your own aesthetic judgement.