If the charge for a drop in visit for a second dog is the same as the first dog, does the sitter visit for 1 hour?

Each sitter sets their own rates and policies. Usually, if there is a separate charge for an additional dog, the time limit for the visit does not change. In your example, hypothetically, the charge would be $24 for a 30 minute drop in visit that would cover both dogs. This is a question to discuss with your potential sitter/client before booking the stay.