Did you call the city to learn what its ordinances are for new water line taps ? see this current one that serves front and rear house likely is also one meter beside being one shared sewer line so it may be useful to know what your city ords are for new connections. If your city for example would allow for the install of second water meter with out actually putting in a whole new line then if one of them says something you are able to share that with them. This information is public so your city inspections desk or public works should have it. (in many places older water -sewer lines that have been in place for a long time must be redone to meet new code when they fail) Just to make sure it may not hurt to have a title search firm assist you in verifying if there is any current easement or not. In some places a city water line servicing one home may cross thru a nbrs land if that is the only way that home can be connected, ( it sounds like at one time this neighboring property was one parcel, but no matter these two individuals can work it out them selfs , it may be different if say your house was at the end of the city lines and there was no other way to run the lines but this does not appear to be the deal) Do your self a favor and take some pics of the side of your property that faces these homes on the chance that someone may decide to begin this work with out your consent then you have a record of what your property looked like before to use in a suit for damages.