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An affiliate of ours has come forward offering to advertise our second website on Adwords.This is gray as gray gets. The affiliate could manage to get the other domain banned since your control would be limited. You can't afford to log into his account from any IP assoicated with yours then they are/could be considered linked.
If you have to question 'double serving' I think you already know the correct answer.I do agree with you here, although Google's definition is "Showing more than one of my ads on the page". The other ad would be the affiliates, so I thought i'd check it out. (Although in the same breath it does say Ads across multiple accounts for the same or similar businesses are forbidden...
Non-unique Landing Page: Google won't show multiple ads leading to identical or similar landing pages at the same time, even if the pages have different domains. This means that if another advertiser's ad leads to a landing page that's similar to yours, and his keyword has a higher Ad Rank, his ad will show instead of yours.
even if the pages have different domains