If the other stand is on someone else's property, about the only thing you could do is talk to the guy and see if you can come to an agreement. Perhaps he didn't see your stand, or maybe he just doesn't care. From his perspective, you're practically hunting on the property line too (only 10 yards off of it).
Question for you: if you shoot something that close to the line, are you allowed to pursue it over the line? The answer to this question might impact how you approach this situation. In other words, if you create a stink, will the other landowner shut you out if you have to track something over the line?
This is always the risk you take when you hunt the very edge of a piece of property (it's happened to me).