It's unclear to me whether this is the case for Rover sitters because there is sometimes a specified second pet rate for dogs staying at the sitter's place, but never for home visits. In my case my dogs are low maintenance—just bathroom, eating, and human interaction.
Some sitters will lower their rate for additional dogs, otherwise yes. If there is no rate for an "additional " dog then that sitter charges the full amount. You can also contact the sitter to ask if they are willing to negotiate, although that may be considered an inappropriate request by some. Still, can't hurt too much to ask. In general rates are set for that sitters' own reasoning.
If you are asking in regards to having your dogs stay at a sitter's house, it varies sitter to sitter If there is no additional dog rate then yes, you would pay double. However if you are having the sitter come to your house, I believe it is a flat rate.
I charge $5 less than my normal rate for each additional dog from the same family. I only accept dogs from the same family, and I realize that they may have more than one. So far it's not been a problem, but I leave it open for negotiation.