How to statement two guys as the joint First Authors

Last week Ma asked me how to declare that she and another guy, both of them, should be treated as the first authors of their paper. I had no idea what was the formal way of this statement at that time.
By chance I saw one paper, which statemented their joint First authors in this way formally:
"The authors wish it to be known that, in their opinion, the first two authors are to be regarded as joint First Authors."
That's it.