A forthcoming paper in Philosophy of Science has dual first authors, Kate E. Lynch and Pierrick Bourrat (I’ve written about Dr. Bourrat’s work previously, which is part of the reason this is on my radar):
Author order has been decided randomly, therefore both authors are first authors. KEL and PB contributed equally to the manuscript. KEL’s distinct contribution was the ideas developed in Section 3. PB’s distinct contribution was the ideas developed in sections 4 and 5 and the equations in Section 3. Other sections received equal contributions from both authors.