Use it as an adverb, a descriptor which modifies a verb or adjective:
Meeting a stranger on the sidewalk, we each stepped simultaneously into the other's path, and then smiled.
Note that the word modifies the verb form, stepped; so the adjective, simultaneous, with the suffix, ly, is an adverb.
Meeting a stranger on the sidewalk, we each stepped simultaneously into the other's path, and then smiled.
Note that the word modifies the verb form, stepped; so the adjective, simultaneous, with the suffix, ly, is an adverb.