IMO, that doesn't make a good trade off. The identity problem space is already super complicated due to ITP while the risk is very high. If you make mistakes, you will accidentally leak personal information to others while the benefits... are pretty minimal, probably having 1~2% more revenue from those people with dual accounts. Given that joining signed-in and signed-out identity is explicitly prohibited as a condition of Double Click acquisition, I doubt if there's any incentive to do this kind of joining for Google.