Fred (Arcadefever) was the specialist for that, I'm remembering his
NINE in one jamma setup
...all the story on his post, right
here.
I used to play on 2 in 1 jamma setup, even back in the days in the local Luna Park (and it was precisely Ghosts'n'Goblins with an other game I cannot remember). No specific problem, just need a good air ventilation as you can guess.