Actually, I've found that if I set up my first route to a planet with the Trade icon, the other trade ships will not open a trade tab while in orbit. What I've done ,after the first route, is set all new routes as I would a harvesting route. I have yet to see any of those ships stop, though I haven't tried to keep an eye on how much each ship is trading(if any). I have noticed that my resources are stacking up a lot faster now!