I wait until they have swallowed the first, and are looking around again. Once, b/c the feeders were chewing their way out of the lil cardboard box from PetCo, I threw the second one in while my cinny was swallowing the first. Instead of finishing swallowing, then striking, he struck with the first mouse's tail still sticking out of his mouth Then, b/c the mouse isn't nearly as flexible as his neck, he had a hard time constricting the second mouse. He eventually got it, but it was really strange, and I can't imagine it was very comfortable trying to constrict with a mouse in his throat...