Different species, different ID numbers
The chance of getting an egg is 49.5% (has a value of 50). The old man in the Day Care will say: "The two seem to get along."
When there are two compatible Pokémon in the Day Care, for every 256 steps that the player takes, the game will decide whether the old man has found an egg
Basically, every 256 steps, you have a 49.5% chance of getting an egg.
Just be patient. It Is A Virtue.