I emailed 2 sitters through Rover, and now can't bring up an option to schedule a meet and greet.
There is not a button to schedule a Meet & Greet, it would just be you and the sitter discussing when you would like to have a Meet & Greet. You can definitely interview more than one sitter at a time! The dates are just required when sending a request through, and this is because you don't want to go through all of the effort of finding a sitter and interviewing them and then finding out that they aren't available for the dates you actually need care.
I would just throw the option for a meet and greet and potential dates right in the message to them =) As a pet sitter, I'm the one who always has to initiate the meet & greet in Rover. Also, I've always been "interviewed" as one of a few potential sitters so do what's best for you!