Jason,
It’s important what are you’ll do with the information about the frequency:

1. If the action tacking after is not critical (just display, or something like this) you can use one C/T as counter, for counting signal edge’s and one as a timer. ("count for a specific time", as Erik wrote).
There is no need both of C/T to be hardware. You can count in software, e.g. via an external interrupt

2. If the system must react rapidly if the wave frequency is not between some limits then you must measure the period (“measure the width”). So, with one edge you can start a timer (C/T in timer mode) and at the next edge to test the timer value and, if needed, restart the timer.

You could let the timer running freely and capture timer value on, let’s say falling edges, but if the timer is resetting on each falling edge you can use the timer overflow to signal that the period is too large, and prevent some errors in measurement.