According to :

I'm surprised the mere idea of well sharing is so foreign to
everyone.

It's not foreign. However, entering into one of these without a firm
legal basis for it is not only foreign, but very unwise.

At least you seem to have a reasonable relationship with him.

Sharing electrical costs on a guess of, say, $10-20/month is perfectly
reasonable if you're both happy with it. Measuring it may be more trouble
than it's worth. You could put it in a repair fund.

The real issue is repairs. You need to discuss with him circumstances,
cost sharing, and what constitutes "shared equipment". That will
be well, well supply lines, pump, control systems, and almost certainly
pressure tank (unless you have checkvalves between it and where his feed
taps off). If you have a water softener that feeds him, it's shared too.
Do you have filters? Do they feed him? Etc.

How things get done if your neighbor can't get access to the equipment
while you're away is also an issue. Do you trust them enough to give
'em a key?

As for the initial installation cost, it's probably too late, but,
perhaps no harm in asking if your relationship is that good.

Now, once you get that all thrashed out, write it down in simple and
plain language (should be less than a page), and each should keep
a signed copy. That will keep things sane in the future.

Note: because of various incidents over the past years, people are
getting considerably more leary of small-scale water sharing. You don't
want to face major lawsuits over a contamination problem you didn't
catch. You'll probably need to advise your insurance company.
--
Chris Lewis,

Age and Treachery will Triumph over Youth and Skill
It's not just anyone who gets a Starship Cruiser class named after them.