Cross selling on a join page is called leakage. An affiliate sends traffic and the sponsor potentially steals the surfer for another site, thus not having to pay you.
Not good business, IMHO
If I'm paying you per signup, it's not a leak. Also remember, that the person is being sold both sites at once on the same join page, they aren't taken to another join page.
And yes, we are buying cross sales (companies with their own merchants only)
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