I can think of just one reason to set A’s dialogue in roman type surrounded by quotation marks: if B were supposed to be uttering A’s lines out loud, physically executing both parts of the conversation. But in that case, when another character comes on-scene you’d have to switch to italics to move A’s dialogue into B’s head or else the visiting character would witness B’s two-way chatter. That sounds messy, and thus distracting for readers. Make it easy for everyone by italicizing all of A’s dialogue just as you would internal dialogue—even when B is uttering A’s lines out loud. I recently edited a manuscript featuring a character with three personalities, and I found that after reading just a few lines of the split personalities’ italicized dialogue, I stopped noticing the italics altogether and just sank into the fictional conversation.