Yes, I successfully helped a nearby neighbor create a profile, mentored her along the way with using the site, including communications, pricing, etc. and soon enough was able to share my rover client with her. By then, I had worked with her as I gave her some of my non-rover accounts.
When a rover parent contacted me and I knew there could be time conflicts with other commitments, I told them I'd like to introduce them and the dogs to her, so she could fill in as an alternate. At the time, it was mostly for repeat day care. Some days we'd share the pups (often splitting the day in half) and I'd pay her. The couple preferred text/photo updates went to each of their cell phones, rather than the rover assigned number, so both of us used those And I made sure at least one went to the rover assigned number daily, even if I had to forward with message indicating it was for Rover photo album. It worked out perfectly.
In your situation, if I worked 8 hours, I would have asked if she could keep them for 9-10 hours for the daycare rate (then I'd be less rushed coming/going to work & not stressed out about what could be happening at my home with canine guests left there, etc.) and I'd pay that out of the boarding rate. Even if it doesn't leave you much incoming revenue, it's okay, because you're keeping the client happy (and loyal to you and the alternate) and building a relationship with the back up sitter, who some time may be able to reciprocate.
I haven't run into that but that's an interesting idea! I really like it. I would definitely consider teaming up with someone to help each other out and keep the dogs well-cared-for. I know when I had my dog a couple of friends co-watched my dog and it worked great.