JAR is right, but in this case, it could go either way--the second utterance is said by one of the simultaneous speakers, so it is a sort of continuation of a character's dialogue. On the other hand, it is different in that they both are not speaking the second line. So I could see it as either one or two paragraphs. If you are going for traditional publishing, it will go through professional editing, so there will be a chance for another input on your choice.
Something separate, though, is emphasis. I presume you have them saying the exact same thing for emphasis, and one way to make that simultaneous dialogue stand out would be to place the following line as a separate paragraph. And while we are on emphasis, any line of dialogue we want to surprise the reader should be presented with the dialogue first and the attribution after. Otherwise some of the surprise of the words is blunted. So, to use the last mentioned example, it would be something like:
Jon'than glanced at Terri. "You already are," they both said in unison.
"And for much more time," Terri added.
If I wanted even more surprise and emphasis at the simultaneous dialogue, I'd eliminate the 'Jon'than glanced at Terri' part and just lead with the dialogue. This would be a good thing to eliminate if you are finding you have a plethora of glances and looks, and squints, and other forms of eyeballing.