A user with Sales & CRM access can set up a relationship between an advertiser and an agency to have a hard link between them. Advertising deals can then be created so that the deal can be accessed from either account. Also, down the road of the relationship, the advertising may be replacing the agency or in-sourcing the ad buying process and in that can users will still be able to view all the advertising that the advertiser has been involved in.
First, when you have located the agency you want to represent an advertisers, set up a new relationship under the Relations tab. Click New and in the window, select the two organizations along with the directional relationship selector.
Now, while still having the agency's client record open, go to the Deals tab and create a new deal. Create the deal as you'd normally do, but this time you also open the Agency settings tab in the deal where you select the advertiser. Here you can also override where the bill should be sent in the end.