Best way to feed when you have one that doesn't eat so well is by separating them. You can make a divider using something they can still see each other through and be able to share the warmth. I don't know what you have available there but I used a piece of hardware cloth wrapped the edges in tape so no sharp edges were visible. That way you can have 2 separate feeders and water containers.
Cut the niacin into 4ths and give one fourth mashed up in the feed for each duckling.
Cut the niacin into 4ths and give one fourth mashed up in the feed for each duckling.