This question has been asked and answered on multiple threads. Here’s one: Sitters applying for multiple sits at the same time
Regarding the ettiquette: See the process through. I wouldn’t mention the other sits unless a homeowner is taking a lot of time. After chatting face to face online with a HO, if the sit is offered to you and you feel comfortable take Let the other HOs know you are no longer available, by courteously writing them letting them know you need to withdraw your application but would love to sit for them another time. Then withdraw your application.