Check out whether the VFS office in your city is allowing you to submit the passports in person, or is still following the two-way courier process. In the case of the two-way courier, click
here. I would encourage you to raise two separate webforms for your parents if you've applied for their visitor visas separately. You can do it one-after-the-other. Upload the passport submission letter they would've received in their gckey. The courier details and information will be sent via email 24 hours after you pay. They'll come to your residence in 3-5 days to pick it up. Follow the instructions given in the email.