I see the rate listed per night for boarding but I do not know if the rate is multiplied by the number of dogs or if there is a multiple dog discount.
You have to expand the additional services and rates are at the bottom of the rates section to see the rest of the rates, if they have a discount it will be there.
After clicking on a siter's profile that interests you, Under Rates, Dog Boarding, there's a link for additional rates, which should show additional dog discount. Some sitters may also mention in their profile. If there are sitters that interest you and you do not see a discounted rate for multiple dogs, you can also message them through Rover and ask if they offer one. I'd suggest you provide as much information as you can, such as dates off stay, and information about all dogs.
Some sitters offer discounts for multiple dogs while others do not. If a sitter hosts dogs from multiple households at once, then a second dog from one household will fill the spot that could otherwise be taken by a client who would pay the full rate for services. Additionally, the sitter doesn't give less time or care to the second dog, so they may feel it isn't worth it to offer a discount.
On the other hand, many sitters find that hosting multiple dogs from the same household is easier or less stressful than hosting dogs from multiple households, since they already know each other and usually have similar routines. Dog siblings can provide each other comfort in a new situation (though that's not always the case). Further, many sitters prefer a guaranteed booking for two dogs from one client over a possibility of multiple bookings from different clients, and that is worth offering a bit of a discount. It may mean fewer meet and greets and less time spent making arrangements for the stays.
As others have indicated, just check the additional rates on a sitter's profile to see if they have an additional dog rate listed, and if they don't, you can always ask!