13 months later, this part of our build project is done.
I drilled a 2.5" hole in the foundations of both houses. Had our grader/septic guy dig a nice trench with his backhoe. Took him less than 2 hours to do what would have taken me a day, or maybe a week, depending on method and tools. It was a great reminder of the value of hiring out the tougher tasks, rather than renting or buying the equipment. Trench is about 200' long.
Ran a 2" conduit between the two homes in the trench. Pulled a 1" Pex line, a Cat 6 cable, two Cat 5e cables, and another pull line through the conduit. Actually, it was easier to push it most of the way: 1" PEX is really stiff. Teed into the existing 1" outfeed side of the pressure tank in the first house. Cutoff valves inside each house, on either end of the line. In the new house I extended the conduit so the PEX exits directly over a workshop sink in the basement. If water ever gets into the conduit, from damage to the PEX or any other source, it should spill into the sink. The new house is downhill from the old house; any free flowing water is going to go to the new house.
Plumbing sub-contractor tied the 1" PEX to the 3/4" main feed in the new house. Good water pressure in the new house. Worked exactly as planned/hoped for.
Haven't hooked up anything to Ethernet yet, but I'm looking forward to networking and Wi-fi working in the new house shortly.
Thanks for the suggestions, everyone! Next up: The existing pressure tank is just about worn out. Going to DIY the replacement.