the younger one will tap on the door with her paw if we're downstairs, or she'll come find us and snap at the air (not near us) to get our attention.

this works very well, but because she knows she sometimes gets a cookie if she comes inside (we live on a large wooded area, and the other dog is stubborn about coming in sometimes) she'll tap on the door all the time even when she doesn't have to go. we just sort of roll with it, it beats her having an accident in the house.

the older dog goes outside when she does, but rarely he'll tap on the door or come stare at you, usually if there's a rabbit he wants to chase