The Data Signal Modulator allows you to choose from:
- MDCIn1
- MDCIn2
- CLKR
- CCP<1:4>
- Comparator C1,2
- EUSART Tx

There are 3 signals that can be modulated; CarrierH, CarrierL, and Modulator. Configuring COMP1 as your modulator signal. Create the 20khz from CCP1, and your 50hz from CCP2. Make one your CARL and the other your CARH. From there look at Figs 23-2 through 23-5 on pages 207-8 of the PIC16F1824_28 data sheet.

Hope this helps.