One star is enuff in a 55 gal...both would compete for the same food.
Altho I'm not partial to Protoreaster species due to the fact that they tend to become predatory when hungry and will eat your CUC and even sleeping fish, your best bet is to keep the starfish fed, which will likely mean target feeding it a variety of meaty foods.
Also, since your tank is FOWLR, be sure to watch your nitrate levels as echinoderms are sensitive to them.
Finally, fish like triggers and puffers will eat sea stars.
HTH
