[billbo911]

Originally Posted by Ether

Thanks for the detailed answer.

I was wondering if you could achieve a similar result using only one sensor instead of two by setting up one pin to interrupt on both rising and falling edges. Or doesn't the hardware support that?

I'm sure it is possible, here is a quote from the Arduino Interrupt Reference page.

attachInterrupt(interrupt, function, mode)....

...
mode:
defines when the interrupt should be triggered. Four constants are predefined as valid values:

LOW to trigger the interrupt whenever the pin is low,
CHANGE to trigger the interrupt whenever the pin changes value
RISING to trigger when the pin goes from low to high,
FALLING for when the pin goes from high to low.
The Due board allows also:
HIGH to trigger the interrupt whenever the pin is high.
(Arduino Due only)

You would have to craft the ISR carefully, but it should be possible using the CHANGE mode. My only concern is that the second interrupt would occur before the processing of the first interrupt had completed.
Although, given the timing measurements we are seeing for this year's challenge, that should not be too much of an issue.