Passing the input from one pin to another as output without polling?
You could use an analog multiplexer such as the 74HC4051 (8 channels) or the 74HC4067 (16 channels) like this:



Arduino B could alter the multiplex selector (A/B/C) pins to route the signal from the test points to a specific pin on Arduino A. If you are wanting to see the result of the signal on a different pin then a second multiplexer would be required (ie. one to send data to a pin, the second to see the result on a different pin).

I have a post about the 74HC4051 multiplexer / demultiplexer which gives more details about using it.

When I tested it I found only about a 5 ns delay in timing:



Ignacio's answer about a crossbar switch sounds like a more elaborate form of multiplexer. Personally I haven't used one of them.