Rover's system will charge the full rate for each dog the customer has listed in his/her profile. However, you can set a lower "additional" dog rate for the services you offer, so that they get charged a discounted rate for the second dog automatically, without you having to make a manual adjustment.
Go to Rates in your Profile:
https://www.rover.com/account/profile...
and click on the +Additional Rates under Drop-Ins.
Just remember, it is very important that all the client's pets are listed on Rover for insurance purposes, just in case the client tries to talk you into only charging for one pet when there are more. Using the Additional Pet fee option provides you with all the necessary coverage.