The small corner-area of my neighbor’s yard borders two other properties: my property, and the property of our mutual neighbor, Sally. The corner is a nice patch of earth where I’ve been growing tomatoes for about ten years. My neighbor has never said anything about my use of the area; he hasn’t even noticed. About three years ago, Sally started planting a small garden there. She walks around my tomatoes regularly as she waters her plants, and I’ve never minded, since she was always friendly. I know that it takes ten years to get adverse possession over a piece of land in my state, and I’d love to “own” that corner. But can I claim possession over that portion of my neighbor’s land if Sally has her garden there too?
In order to obtain title to a piece of another person's property through the legal doctrine known as "adverse possession," one of the many requirements is that the trespasser have exercised exclusive control over the area of land in question.
This means that other people cannot share the same piece of land. Indeed, the trespasser must act in a manner consistent with ownership, as if he were the true owner of the land.
Here, you seem to be sharing a small patch of your neighbor’s land with a third party. Acting in a manner consistent with exclusive ownership of that patch of land would probably have meant that you should have asked Sally to leave (or sign a lease agreement with you) when she began gardening there a few years ago.
If it was a larger area, perhaps you could try to argue that you claim adverse possession over the left side of the corner, while Sally would claim adverse possession over the right side of the corner (or at least in seven more years, when she has met the statutory requirement).
But it sounds like the patch of land is very small, and she was often on “your side” by the tomatoes without any objection. Most likely, you will be unable to meet the requirement of exclusivity to gain title to a portion of your neighbor's property by adverse possession. Consult an attorney for a full analysis of the facts and law in your state.