The Best Joining, -if Aggreeable to all Parties, -would be from Well A as the Source, -but "Post-Well". This means simply a Common Surface Water System using Well A as the Initial Source. A Ground Based, rather than a Subsurface based, Solution. If by "Joining the Wells", -you had been Thinking of some kind of Subsurface Joining.

I don't know much about wells, easements, or legal agreements, but a lot of practical experience tells me this shared well project is a very bad idea! No way would I get involved with neighbors in this kind of arrangement. Finding a good fencing contractor and erecting a nice high privacy fence on the property line seems like a better idea.