Andres,
Unless you need barriers for some other purposes, you don't need to use barrier costs to limit the search to only the closest neighbors in the OD solver. You should be able to achieve this by setting a Target Destination Count (e.g., find the closest 5 for each origin) or by setting a cost Cutoff value on your search (e.g., only search up to 10 minutes away from each origin).
However, even if you must still use barriers, if you are using point barriers with additive cost, then it probably won't matter too much whether the barrier points are located right on top of each other versus located just next to each other on the same edge. This is because the cost to go through these barrier points will still add up to the same amount (assuming you do need to travel through each one). Are you asking to disperse the barrier points or your other origin (or destination) points?
Does this make sense? If you can further clarify your use case, perhaps we can come up with some better alternatives.