It's usually best to have to guest with status make the reservation in their own name.

As mentioned, you can pool points for free - if not immediately then in the near future.

AFAIK, any effort to get Diamond benefits as a second listed guest is down to the generosity of the hotel rather than a published benefit. This, among other reasons, is in case the "second listed guest" doesn't actually show up and is just trying to pass along their benefits to somebody who isn't otherwise entitled.

The extra benefits of Diamond vs. Gold are limited, but you could also just leave things in the name of the Gold guest and do a DYKWIA if you don't get lounge access or whatever extra you might be hoping for.