I think what you are referring to as noses are the two pairs of tentacles that slugs have. They use these tentacles to gather information about their environment. The pair of tentacles located on the top of the head has a small black spot at each tip. These tentacles are used to detect lightness and darkness. I am not sure if they also used for smell.
The second pair of tentacles is located at the lower part of the head and functions as a nose, because they pick up chemical smells. They are also sensitive to touch. To find food, a slug uses all four tentacles.
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