Connecting Ad Buying System Across Websites

Hello,

I wanted to see if anyone had any recommendations about how I might create an ad sales system, potentially using plugins like this or this (but not necessarily), which ties together two or more domains.

I have two domains, say abc.com and xyz.com, and I want a client to be able to buy ads on both sites, automatically, without having to create separate accounts or entering their credit card/paypal information more than once. Is there a way of creating a single log-in space where they can enter and purchase ads on both abc.com and xyz.com?

Any proposed solutions are welcomed and appreciated. Thank you.