You will need to create a new Pet Profile for each additional pet. In the upper left-hand corner, you will see your first pet’s name. Click on the small downward-pointing arrow to the right of your pet's name, and an option will appear to "Add a new pet." See below.
After creating a new pet profile for this next pet, you will need to register the 6-character sample ID code to them, just like you did for the first pet.