Disagreement is a healthy part of business. Opposing viewpoints need to collide in order to create the most fully realized perspectives. However, eventually, people need to come to an agreement. Lew Sauder discusses how to get there in a post at Consulting 101.
In the first place, clarify that all parties agree on what the actual problem is that they are trying to solve. It is no wonder if people disagree if they were never on the same page in the first place. Second, consider the different assumptions that people are working under that inform their views. Look for assumptions where both parties agree. Then try to build around that. All sides will likely have to give a little in order to reach a worthwhile compromise, which will be a painstaking but unavoidable process.
Whatever is ultimately decided, make sure you get it down in formal writing. You can view the original post here: http://blog.consulting101book.com/getting-to-agreement/