Is this still a 55g tank? PLEASE PLEASE PLEASE do not add another tang. The yellow is plenty and many would argue it is not appropriate. But I definitely say "no" to two. Just a bad plan, IMO. If small the yellow may be OK for a few years.
Please feed your tangs more, IMO, than flakes and nori sheets. Look up some frozen formulations for herbivores. There are several that have a large component of algae. All fish really need this diversity. Flake food is not sufficient.
How old is your tank, and what are your specific water parameters? How long did you acclimate the fish?
Chocolate chip stars, IMO, should be spot fed and not left to eat algae off the glass or scavenge. Please consider giving it food, at least once a week if not more frequently. Also know it is not reef safe and is a threat to your clam, snails, corals, anemones, etc, etc.