Anon. has left a really insightful comment in the post below, well worth reading. Here's my response.
Thanks so much for this insightful comment. There are wonderful stories of good relationships among colleagues working on a staff together, but of course, those don't get heard as often as the horror stories. And every job description should have some of "shiny" parts of ministry, the creative, public aspects, and some of the "heavy lifting". Also each minister (really, this goes for any staff person whose job description can be negotiated) should have some things that really excite them. My experience in working with a number of colleagues is that it is hard to know, in advance, what that split of work is going to be and also that it changes with time. I think that it is best for church leaders to give the two ministers as much leeway as they possibly can to decide between themselves how they are going to split up the ministry and to expect them to tweak their division of labor every year. Much as they might like to only hear the senior minister preach, for instance, if the Associate likes to preach and you want a happy associate, you have to find ways to let that happen. (You do want a happy associate and a happy team. An unhappy minister is a menace to themselves and their church and a ministerial transition is a huge expense.) If all ministers have some basic HR training, (that includes training in how to be a good senior colleague) they will know how to negotiate and re-negotiate these things and why it is important to do so.