Serving as one of these misplaced pastors is an odd experience. It gives one a distinctive third wheel feeling when one is to read the Gospel lesson, for example, but just sort of hang out over at the left end of the chancel while the celebrant leads the rest of the service.
But there is no need for this to be either confusing or stressful - the assisting minister should simply vest and serve as a liturgical deacon. Of course, if yours is one of the few parishes that is blessed with a real live ordained deacon, all the better! But it is appropriate for any pastor to serve the deacon's liturgical role when he is called upon to assist in the Divine Service.
Detailed rubrics for such a celebration with deacon, and even subdeacon, can be found in the edition of Piepkorn and McClean that Fr. Petersen caused to be published.
A deacon is most appropriately vested in alb (and amice), cincture, deacon's stole, dalmatic, and maniple.
I have found that even pastors who are unfamiliar with a liturgical deacon's role much appreciate being involved in the service in such a thought-out and dignified manner - as more than an afterthought. We join with two other parishes for Ascension and Epiphany services and keep a set of vestments - dalmatic and tunicle - for these occasions. A copy of Piepkorn/McClean and a matching set of chasube, dalmatic, and tunicle would be a fine addition for any circuit that regularly gathers together for such services.
And for the parish with two or more pastors, contrast the unity and beauty of this manner of conducting the Divine Service with the simulcast approach becoming more and more popular in certain circles. In this model, two services occur at the same time, the second beginning some 15 or 20 minutes after the first service. This allows the preacher to move from one worship space to the other.
Such a procedure is American in more than just its efficiency. Lost is the idea of the Divine Service as a unified act, a pressing together of the whole people of God in that place toward a common goal: receiving the blessing of God together, as community. The sermon, and the bearer thereof, is very much the cog dropped into the machine at just the right spot and time.
In sum, making use of multiple pastors by using the traditional liturgical roles of celebrant, deacon, and subdeacon adds a sense of unity, beauty, dignity, and reverence to the Divine Service.