Hello,
I tried to search for it, but could not find it. How can I test many ads on TJ? If I create two ads for a single placement, does that split them? What is the weight thing? Any help for that?
Thanks
It splits them based on your weight you give it. So if they all have a weight of "1" - then they all get displayed evenly (theoretically).
And if they are on the same site, but different placement? Do they also split them or they show both?
How come they don't have a guide for that in TJ? Or is it hidden? 
Not sure about the guide.
And don't remember offhand exactly how TJ works regarding placements and campaigns, but basically - its per campaign. You add your banners, and any banners you add will be split. It defaults to weighing them at 1, so it would display them evenly.