Add the shells gradually to 6 quarts of boiling salted water and cook until tender, yet firm. Drain; rinse with cold water, then drain again. Set aside, upside down to cool.

Combine the crabmeat, shrimp, cheese, salad dressing, celery, onion and pimiento in a small bowl. If the mixture seems too dry, add more salad dressing. Spoon the mixture into the cooled shells; cover and refrigerate until chilled.

Serve the shells garnished with celery leaves.