I said you always want to try to handle it diplomatically to begin with. Try to have them see it from their co-worker's point of view before a decision could be made. Each and every person comes from a different part of life. So, each and every person will see a different approach to reach a solution. As a project manager, you have to teach them to walk in each others shoes for a few minutes and she where they are coming from. When that happens, they begin to step back and compromise on a solution they are both happy with.