Both Martha Plimpton and Clifton Collins Jr. have joined “ The Lowdown ” Season 2, Variety has learned exclusively. The pair are the latest new additions to the second season of the FX series, alongside Tommy Lee Jones, Betty Gilpin, and Austin Amelio. Ethan Hawke stars in the series, with the Season 1 cast also including Keith David, Kyle MacLachlan, Jeanne Tripplehorn, Tim Blake Nelson, Ryan Kiera Armstrong, Michael “Killer Mike” Render, Kaniehtiio Horn, Tracy Letts, Peter Dinklage, and Graham Greene. Exact charater details for Plimpton and Collins are being kept under wraps.…