It's not like Dave doesn't trust Terezi to act rationally, even if trolls are fucking nuts. It's Rose he doesn't trust at this point in time. She looks just like his sister and acts like her for the most part, but there's a huge degree of separation between them. This is why he fucking hates timelines, this is why he hates it when he isn't the one ahead of the game.

"What? Nope." He makes a face of confusion when he turns back to Rose, brows knitted together at the order. "You need to back up a few steps and take a breath, Rose. We both know Terezi, don't we?" He's staying stubbornly still in his place between them. Again, it's not because he doesn't think either of them could hold their own, but he really doesn't want a fight to break out over a misconception.