We have a different experience from this. In 2018, we cruised as silver castaway club members with my parents who had never sailed. We were able to book excursions for both parties when the silver window opened, as long as someone from the silver group was on the excursion. We could not book for just the new cruisers if we were not part of the group. This was an Alaska cruise, and I was able to book for myself and my son (silver cruisers) and my mom (newbie) a whale watching excursion, and book fishing for my husband, other son (also silver) and my dad (newbie) at the time Silver window opened.
I was not able to book Palo for just my mom and dad until their window opened, since we were not going to Palo at the same time.
So, I do believe that you can book together as long as you are doing activities together. But as @lizzieanne771 said it would be best to ask shoreside concierge to be sure. And the reservations would have to be linked. And the non concierge group would not be able to go to the concierge lounge or special activities like she said. We also travelled with my sister staying concierge and were not able to attend any of their concierge level special activities (not that we even tried, we knew we didn’t pay for those).