you need to use a selector device... both pins would connect to that device... either "pin a" or "pin b" could use the sub, but both could not use it at the same time...

for a myriad of reasons, hooking two amplifiers directly up to a single speaker would be a very bad idea... unless you really like replacing amplifier/sound boards...

It depends on the sub and what inputs it provides. I (and many other Pinsiders) have a Polk PSW10 with two pins connected to a single sub. You will need to run the speaker wires from each pin in separate input channels on the sub, one into the left, one into the right.

There are pictures in this thread:
http://pinside.com/pinball/forum/topic/polk-psw10

If you're hooking both up to the external speaker inputs (one going in left, one going in right)on the amp then you should be ok. I would imagine it would sound a little sloppy with two different signals at the same time, having the crossover tuned lower would probably help minimize that.

You can have 20 pins hooked up to the same sub, but you need an interface like a DI box and a Line Mixer to safely isolate the amp circuits from each other.

I have a client who has 6 pins running through an 18" sub. It rocks hard.

Is there any advantage of using the pinnivator $35 adapter when sharing a sub between 2 pins vs. just following this approach?