In my 2008 experience, out of 7 stays with several rooms booked under a single reservations, I had varying results w/r/t Diamond service.

In 4 case, upgrade were offered for both rooms, in two case for 1 of them, (and once no upgrade) in all case, lounge access was offered for everyone. At Hilton Tokyo, were I stayed twice, I was upgraded to executive floor for all rooms once, and for only mine once, so it's not linked to which property you are... I saw no consistency. I didn't expect the benefits to be extended to all rooms, but I can see the point of it (if a family is travelling, it would be silly to grant access to the lounge only to the card holder, not wife and child).

I was, overall, satisfied with it. All the stays were in Asia (Singapore, Kuala Lumpur, Tokyo, Osaka, Dubai).