There has been a Berzerk/Frenzy multboard out for years from Mark Spaeth. Also available at KLOV (or at least was). I have one in my Berzerk. Works great. You switch between games by pressing both start buttons at once.
Mark's board is a straight ZPU replacement running the actual game on the actual hardware (other than the modified ZPU itself). Drop it into a Berzerk and BAM now you have Frenzy too. 30 second install.