Haven't seen one, but Goog has

:
http://www.andys-arcade.net/store/index.php?act=viewProd&productId=1752 ~$50
"Here is an interesting multi-jamma pcb system made by R.S Coin in the early 90's. This pcb allows you to plug 4 Jamma PCBs into one cabinet simultaneously. You pay for time rather than for credits, the bonus is, in the middle of any game you can switch to one of the other games at the press of a button. Instead of a standard marquee, this one has a dual LED digit display for credits and time remaining, as well as one for annu8nciating the current game selection. This pcb actually speaks to you when you insert a coin to select your game, when your time is running low it will speak to you again telling you that you will need to put in more coins to continue your game, very clever stuff, not just a dumb switch!
You will need Jamma extension leads to reach your 4 game pcbs as well as a second PSU which plugs into the molex connector on the game pcb itself. In this way, all 4 pcbs are powered up simultaneously and the pcb allows the switching between them.
I would reccomend this system for more experienced arcade collectors as it will require some wiring up and configuring of additional buttons, as well as jamma harnesses and a seperate switching psu attaching. Please make sure you are aware of this before purchasing, there is no documentation for this system online and you will have to figure out some of the functions on your own, no returns please.
Tested working original PCB multisystem, this is not an out-of-the box multi-jamma solution, it requires some skill to setup correctly"