Not good. You loose another 25-30% of capacity due to the upper and lower pipe stealing heat from each other, compared to a single circuit . You already loosing with the thicker pipe. Adding it all up you loose all your safety margins, and possibly more. Heat is coming mainly from below in the winter, and the lower pipe will take heat out and will allow lesser heat to go up to the upper pipe. Plan on going with 6 trenches.......7ft deep, one pipe flat on the bottom.