Realistically, real separation is only a few inches deep. The farther the targets are from each other the better. As you go deeper you start to get a blend, as the signal goes from two, merging eventually into possibly one. The VDI will be affected by the other target, the reading on the graph, if you have one, will start to go from two to a singularity, and eventually they will start to seem like one. You'll get your information from sweeping carefully, and then sweeping from a different angle, 90°. Coil control, pinpoint, another program, use every tool you have that the detector offers. The audio may give you the clue, or the graph. The only way to really know is if you get enough information that tells you to dig. That tells the story. Seen many times people digging up something gold, right next to a bent square nail, or something similar. That's where the concentric coils shined, they sort of have the ability to nail both objects a little better than a DD coil.