Can't speak from first hand experience, not being a prostatectomy patient.
As part of the prostatectomy, your internal urinary sphincter was removed, leaving just the external urinary sphincter for urinary continence which is part of the pelvic floor muscle, as is you anal sphincter too. To fart, you relax your anal sphincter. That is now part of the same muscle which provides urinary continence. They can be controlled separately, but you never needed to before as the internal urinary sphincter (which you lost) would maintain continence. What will happen now is that you will over time learn to control the external urinary sphincter separately from the rectal sphincter. It's like raising one eyebrow, a level of control many people don't need, but can learn if it is needed. I think there are some specialised pelvic floor exercises to help with this, but it is different than just the basic pelvic floor exercise.