Well, not to each other...
Platies are live bearers. Most breed readily as long as an appropriate gender ratio is present. Raising the fry depends on providing adequate hiding places, or removing the adults and other fish that may eat the fry.
Danios breed readily--it's getting the eggs to avoid predation that can be a challenge. What works best is to have a tank with internal filtration (gentle flow) and large rocks or marbles for substrate. Condition the fish in another tank, then move into this tank for a week or so. Remove the adults, and then monitor the tank for wrigglers.