a!writeToMultipledatastoreEntity

docs.appian.com/.../Write_to_Multiple_Data_Store_Entities_Smart_Service.html

Well, considering this is its primary use case, I should say so.  If you're having issues, you might want to post a more detailed question / what you've tried and what problems you're running into.  If you're just unfamiliar with it, I'd suggest reading the documentation Stefan already linked.