pic 12f675 - pwm counter

I am working on a project that requires a pic 12f675 to measure a generated pwm on one pin and output a certain ammount of pwm on two other pins.

input>
0% - - - - - -50% - - - - - - 100% > duty cycle
|---------------|---------------|

output >
100% - - - - -0%- - - - - - - 100% > duty cycle
[---on Pin A--][--on Pin B---]

Because I am using software to generate the output pwm the code that measures the input pwm will have to be in some sort of interupt or timer so that it won't affect the the pwm being generated. The smallest amount of time that the code jumps out of the pwm routine the better. Maybe I could run the pwm in an interupt or something. I am really weak with interupts and timers so this is where I need the help.

It would probably be a LOT easier to choose a PIC with hardware PWM.

Such as the 12F683, which is basically a 12F675 with NanoWatt technology, twice the memory and CCPWM.