Hi! This is what I do in order for my beta to eat his food while my tetras it their food.
First, bettas can jump out of the water. Thus, whenever it is his turn to feed, i put a bit of food (i feed him pellets) on my finger and hover it one inch above the surface of the water. That way, he just jumps up and gets his pellets.
For my tetras, I just put their fish food (i feed them flakes) below the water line and they will always come up and eat them. The first time I did this, the betta tried to taste a bit of the flakes and spat them out in disgust. Thus, he never eats them and leaves the tetras in peace to enjoy their food.
When it comes to brine shrimp or other treats such as bloodworms, I put the treats into the tank however, usually the tetras are fast and gobble them all down before the betta has a chance to eat them. Thus, I do the jumping thing again. Put some of the treat on my fingers and make the betta jump for his food. Simple as that. Hope that helps!