Unfortunately, it would appear that the unlimited grenade option itself no longer works. I have it enabled, and I'm running out of grenades.
Edit: Ah, I see. It only works for one of the secondary weapons, not both. It appears that the first selection in the loadout order receives the bonus. If you want to give the bonus to the item currently in the #2 slot, you need to disable the infinite grenades option first and completely change the secondary weapons loadout first, as even if you swap slots the bonus sticks with the weapon that was originally in the first slot - even if that weapon has been swapped out afterwards.