I agree with Shawn.... "or" simply means both is possible but not both is required! And doing something voluntarily is the same as giving your authorization legally
As Fryman pointed out, there is another "if authorised" in the end which makes it a little ambiguos. I think this might mean that sites that engage in this need autorisation from Google unless they are revenue sharing with sites that already have authorisation from Google. So, if two sites (or accounts) are revenue sharing at least one of them need authorisation. I "think" that this is what it means anyway.
It's not the best grammar, but the second 'if authorized' refers to the adsense customer. It says that google needs to authorize, or the adsense customer needs to authorize. If they left out the second if authorized, then it would ask the question about if the adsense client would be able to give authorization.
Shawn, Would it not be worth contacting Google and asking them if your revenue sharing scheme is still abiding by their rules. Even if you feel that you are abiding by the rules it will help these people who are paranoid about getting their adsense accounts banned have peace of mind.