How to find duty ratio of a PWM using input capture?
My understanding is that a CCP module can capture either a rising, or a falling edge, but not both.

To get the duty cycle you will need to capture both the rising and falling edges to then compare the times of both and work out the ratio.  That can't be done with just one module.

However, there's nothing to stop you sending the signal in to two separate CCP modules, one configured for rising edge, and one for falling edge, and using the two to calculate the difference between the rising and falling.

You'd basically end up with two periods, which should both be the same, but a phase difference between them, and it's that phase difference that would equal the duty cycle.