switching sensor output between two arduinos
I would suggest using a tri-state gate. A tri-state gate is a digital logic device (available in IC form) that basically allows you to disconnect a circuit element from a circuit electronically. In other words, the tri-state gate is much like a mechanical switch, but instead of flipping the switch, you control the gate with an electronic control signal, like an arduino pin output. When the arduino output is off, the tri-state gate is open, meaning that it looks like a break in your circuit. When the arduino input in on, the tri-state gate looks like a buffer, allowing digital 1's and 0's to pass unimpeded. Google these devices to find an IC that suits your needs, and from there learn the specifics of the device you've selected.