Occasionally adding an extra pup to a reoccuring walk?

I just booked my first client and they would like reoccuring walks on Tues/Thursdays for their golden! Everything is going great, but there is sometimes a second dog there that they would like to be walked as well. I'm unable to find how to add the additional dog and charge to just one of the appointments instead of all? I know it should be under the Modify options, but all I'm able to do is change the time or add a whole second session