I don't have any experience with DX, so I'm trying to imagine what the behavior of the vaporized refrigerant would be, based on what I have witnessed so far...
I initially imagined that there might be a condensation problem, but then I realize that condensation will not happen in the low pressure side. Another aspect would be that the temperature of the liquid-flashed-to-gas that is exiting from a high-pressure tube into a low pressure area will be the very lowest in it's trip back to the the compressor, so the delta-T will be greatest and the heat migration into your vaporized refrigerant will be maximum at that point. The delta-T will decline as the vaporized refrigerant gathers more heat. So there could be an advantage to having all that take place at the deepest, most wet, most temperature-stable part of the bore hole.
The opposite scenario would be that if the liquid-to-gas transition happened at the top of the hole, the vapor would have a longer distance, and longer time to gather heat. The construction and maintenance would be much easier. Also, if you went for a TXV, it would need to be at the top of the hole. But I think that a TXV for such a tiny compressor would be just too much techno for what should be a simple, elegant project.
I would lean on the second scenario, with the cap tube before the loop. While the subcooling of the high pressure liquid refrigerant could help superheat the exiting vapor to above that of the ground temperature, the heat gained would not be stellar. Most of the extra heat would be radiated into the ground anyway. Giving the liquid/vapor mix more distance to gain ground heat would be much more beneficial. Remember, liquid moves slowly, gas moves fast through the system.
In the
IGSHPA manual (CLGS Installation Guide (#21020)), in the section on the loop fields, it looks at various ways of doing loop fields, both trenches and boreholes. There was even a small discussion on putting two loops in the same hole, and I believe that the performance increase was about 3% to 5%.
I'm afraid I don't understand how two loops would remedy the lubrication issue?
-AC
P.S.: This manual is a tad expensive ($45.00), but it is so amazing in the breadth and depth of information regarding Ground Source heating and cooling, that you should seriously consider getting a copy. It can sometimes be found used, sometimes in libraries. But considering the information, it is a miracle. As I recall, the manual does not directly address DX, but most of the principles are the same.