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.