Steven does give him what he hopes is a reassuring smile. The spells are certainly safe to use together, he just doesn't have all the data he needs to know all the ways they affect each other, that's all.

Then he refocuses, lifts his hands, and starts sketching out the lines of the spell in the air, dropping a word-- Egyptian, because honestly, why not?-- at each point until the spell Bless coalesced into a light glowing mist that wrapped itself around Donnie.