Is it better to schedule two different drop in sitters than one twice a day?

I think that it is always good to have a back up sitter. In case your regular sitter is unavailable. However, I am not a fan of splitting the services. I think the best solution would be to meet with a few sitter and see which ones you feel most comfortable with and will fit your needs. It sounds like this is going to be an on going need so I would ask the sitters at your meet and greet if all goes well, if they are going to be available long term (or for however long you are expecting) and how would they handle a situation such as if an emergency came up? The sitter you choose may have another sitter they work closely with. But if not I would say to have a primary sitter that you always use then a secondary sitter that you felt comfortable with in case something was to pop up.

I see nothing wrong with having 2 people doing a drop-in visit during the day. You have one person for the first visit, the second person for the second visit. Absolutely nothing wrong with it. Trust me, people can get ill real fast. I am the perfect example. I ran my private dog walking business, had 7 walks every day, that's 7 miles a day and then I got ill. And I mean so ill, and I wasn't able to work for 12 weeks. It was difficult for me, but it caused problems for all my clients too because they didn't have a backup person.

Most Rover sitters won't provide service to you if the service is shared with another sitter or family member just in case something goes wrong. If you have a current sitter I would call and discuss your needs with them