I believe digging the trenches deeper than required for the septic lines would interfere with the horizontal movement of the liquid waste.  Septic lines are spaced a certain distance apart for a reason.

I think his point was the Geo system would be installed at a deeper depth then the septic system, the trench would be filled in to a depth that is the normal installation depth for the septic system, so the septic system really isn't affected. I peronally don't see a problem, but I'm far from an expert in this area. My only concerns would be if the geo loop ever had to be services/replaced, the septic lines would have to be replaced as well. While you might not need to touch them for 50 years, sooner or later the costs of replacement is going to be a lot more. My other concern is if the closed loop isn't big enough and makes the ground colder than it should, what impact would that have on the septic system? If the ground froze, I would guess it would have a major impact on the system, frozen lines, damaged pipes, since it wouldn't have the same kind of anti-freeze protection that the close loop would have.