I faced this problem on two similar occasions, after my long-term coauthor Val and I completed our joint projects. On each of these occasions, we had a lot of new material on our hands. So we could easily afford writing two separate papers, to be published back-to-back in the same issue of a journal. In the first paper, the order of authors was "Michael and Val", in the second -- "Val and Michael".
Do you think you can convince your collaborators to split the paper into two ones?
Things change when the coauthors' job situations are different. Say, when a tenured scientist is collaborating with a junior colleague lacking a permanent position or with a colleague surviving on soft money. In this case, it would be gentlemanly of the tenured coauthor to yield the first authorship to the non-tenured one.