There are many reasons why having more than one firm working on your case is a good thing.
First, there are more resources available. Resources can be in the form on man-hours, attorneys available, or money for investigation.
Second, another set of eyes reviewing your case is helpful. To see the case from a different angle is a huge advantage. We all have different opinions.
Third, it helps spread the risk. This speaks more to the resources available. To investigate and gather records costs time and money. If 2 firms share in this cost, it frees up resources for other parts of your case.