I strongly believe all of the rates, including overnight should be all inclusive of the pet services provided. If you're House Sitting a pet overnight, the rate should include walking, playing, feeding, giving oral medication as needed, affection, etc. (such as light grooming-eye/paw/sanitary wipes, brushing, drying after rainy walk) for approx. 24 hours. If it extends much beyond 24 hours, then it's reasonable to add a daycare charge. If someone has more than 1 pet (whether that's another dog, cat, or fish, etc.), then it's reasonable to charge more for additional pets.
Although certainly you could negotiate prices for other tasks such as plant care, I'd advise that you keep in mind here may be many others who would not. I've done lots of extras (often without being asked) while house sitting because I liked the dogs and wanted their family happy. If I'm not comfortable doing something, rather than charge for every little item, I'd just tell the person, I can care for the pets, but I'm not comfortable doing that (fill in the blank- caring for plants, etc.) In order to "care of your loved one as if it were one of my own", most dogs need walks and 3 per day is normal, not excessive.
As for booking multiple overlapping overnight stays, no, I don't have any advice because it goes back to my thoughts that if you're providing overnight pet care in someone's home that you're there with the pet. To even consider something else, you'd have to ensure all pet owners involved understand and agree to other proposed care arrangements.