I'm not sure it will be necessary, but it could happen.
To my knowledge, the Perry case in front of Lamberth is the only one that has the companies themselves as the defendants. The rest of the cases are against one or both of FHFA and Treasury. FHFA hired Milbank as a legal advisor, and Treasury has the DoJ to advise it. So I don't think FnF will have to hire their own legal advisors.