Specifically, because while that part has two CCP units, one of those must be used to measure the cycle time. So only one is available for use as a PWM.

A CCP unit (or its equivalent on the AVR) is basically a register with a count value, and hardware to compare that count value against some timer. It takes an action when that comparison is true--either to set or clear a pin (for a PWM), or just to set a flag in software (to measure the cycle time).

It's not possible to do two frequencies, even if both are 50% duty cycle. What is programmed into a PWM controller is not the duty cycle and frequency, but the divisor (from some faster clock to the output clock), and the on- or off-time, in units of periods of the faster clock.

LDmicro handles the conversion from duty cycle and frequency to those values automatically, and warns if the requested frequency is not achievable within a few percent.