This is a sizeable concern for players that like to split up.
In a multiplayer game with 2 players, it's impossible to engage in conversation with an NPC while another player (elsewhere on the map) is on the trading screen with an entirely different NPC.
If my friend is trading with someone and I try to speak with an NPC across the map, I will be shown a screen that fades to black. The screen will continue to be black until my player friend completes his trade. When the other player quits the trading window, my black screen fades back in, and I'm allowed to continue conversation with the NPC.
These are two completely different actions that players should be allowed to engage in independently of one another.
Thanks!