Not without a larger space / more seats for the same number of people. Whether it's the whole restaurant or half of it, the problem is the same.
If you have 800 people to feed on one level of the restaurant, and 400 seats, you can do it easily in two 400-seat sittings. If you have open dining and only 200 turn up at 6pm (because early, is too early) half the seats sit empty for some time until most people (let's say another 400) turn up sometime in the middle of the evening (because early is too early, and late is too late) but they can't all be seated because some of the first to arrive are still eating. The only thing to do is to tell some of them to come back later. When they do, along with the last 200, there are still not enough seats because some of the 400 mid-evening arrivals are still dining. You can't really tell them to come back earlier. Because those seats sat empty at the beginning of the evening there will never be enough seats for everyone.
The only solution is more seats (or fewer passengers), which could be designed into a newbuild but won't work on the existing ships, unless they can somehow squeeze more seats in or increase the size of the restaurant.