Personally I've been unable to establish more than a single trade route with another players planet. I set up the first one, then any time I try to set up a second I don't have the trade option. It's highly obnoxious.

On the subject of trade routes, I'd LOVE it if there were a way to see if you already had a trade route (or multiple routes, if I could actually create em) established with a planet. It wouldn't be so hard to add the same image as the harvesting uses - the top left section, you select it and see all ships with a trade route to/from that planet. It'd be a huge help for figuring out where you already have routes set up without slogging through the AI ships tab.