Daniel, in your example my opinion is that the second affiliate should get the credit. Sure, place a cookie but make it so that it is overwritten if the player comes in subsequently from another site. This avoids the unscrupulous(?) practice of using methods to open the casino without the visitor knowing and the cookie being placed. Using 1×1 frames and other methods have been used in the past.
This was a hot topic among affiliates back 18-24 months ago I think. Anyone else please correct me if I have got this wrong.
The life of the cookie is also another consideration, even if the player never comes in from another site.