Using a "second team" or Reference Group can be the single most important step you take to build in business case credibility. 

Business Case Credibility is not a Do-it-Yourself Project

The moment it is clear that decisions or plans call for a business case, an insidious, deadly temptation appears. It targets the project manager, product manager, consultant, or sales person responsible for the case. However, even CFOs are not immune to the temptation.  The temptation sends this pernicious message.

Do it Yourself.
Or,
Do it entirely with your own staff.

The temptation has a seductive logic:

If you build the business case yourself, you can be sure to end up with the right results.

And, If you bring in outsiders, there's no telling what you might come up with.

And, it isn't easy to get the time and efforts of people outside your own group.

After all, what's in it for them? The tempter says:

It's better, to do it yourself." Read more of this post