Yes there is a concern that the glaucoma will appear in the left eye.  That is why your veterinarian placed your dog on preventative medication.  The chances depend upon what type of glaucoma this is--there are primary and secondary causes of glaucoma.  Primary glaucoma (also called hereditary or breed-related) is due to an anatomical abnormality.  This abnormality can be present in both eyes.  If the glaucoma is secondary (related to an underlying disease like diabetes, a lens luxation, uveitis, and others) it is most commonly unilateral (affects only 1 eye).  Secondary glaucoma can affect both eyes if the underlying disease is not treated or affects both eyes.  Ask your veterinarian for further information about your dog's specific case.  If you got this dog from a breeder ask if their lines have a history of glaucoma.  Cocker spaniels in general are a breed predisposed to glaucoma formation.  There are veterinary ophthalmologists with whom you can consult about your dog's glaucoma.