I know this is a little unusual - you being a slave to another slave - but she'll follow my instructions and if you follow hers then there shouldn't be a problem. If anyone asks, she's your Mistress.
I've given the shopping list to Slut, I want you to be back here with the list complete by the time I get home in three hours. That should be more than enough time to get the shopping done.
Now I'm going to go back to work - remember to obey Slut, and report to me if she does anything she shouldn't.