If you plan on using both L and R for 3 x 2 pinballs, total of 6, then I suggest you get a device that has independent grounds. If you don't, and plug 2 pinballs at the same time in an input's L and R , then you will be joining the speaker wire of 2 pinballs together and if you didn't wire properly you could be joining one pinball's ground plane to the others output audio signal. Now just like this it might not do anything special, but it might create a ground loop, if the grounds are tied together through the earth ground.. It's just not a great idea. You would still need to make sure that the ground isn't shared at all inside that selector switch.
While searching for your device, I thought I had found the right one, but they differ. Anyways this one seems suitable :
This one is more expensive, but seems better quality too. One important design feature of this one is that : >>RCA terminal and signal switch are directly soldered on circuit PCB board, no wire is used, *** the left and right channel grounds are separated ***, the aluminum shell for shield, and audio signal lossless
Meaning every jack is totally independant from the others, so it actually gives you a possibility of having 6 pinballs connected to the same subwoofer, if that subwoofer has 2 speaker level inputs.
I am not an audio expert at all (I am an electronics tech for the past 20 years though), so I might be wrong and maybe there aren't any major risks of mixing speaker wires from 2 different pinballs together (it SHOULD be fine if you are correctly using the ground from both machines), but I would really put extra care on that. Don't cheap out on this aspect if it can potentially damage your pins or sub..