Does the second one heat?
Who is someone? and what are his/her qualifications?
Normally two hws are hooked up in series, that is to have a common supply & common outlet both flowing in the same direction (manifold). However I have seen multi gas hws in series(manifolded) supplying a storage tank then to the close by fixtures, you say that you have the hot water distributed to eight units, Have you got a circulating pump? if so then someone could be correct. if you have dead leg lines to the units then you probably are not even getting hot water to some of the units, especialy the farthest units.