It would certainly help .. if you put them in tandem (linked together) then be careful as the feed from ball cock must go on 1 tank and the draw off the other so as to not get stagnant water!!
Why didnt they exit to the rear or if its a single story extension they coulda gone vertical :roll:
At the end of the day its wrong so speak to the neighbour ASAP with your findings :wink:
Thats the diverter valve !!
Isolate the flow and returns under the boiler then run the water ,, if it gets hotter then its gotta be the diverter,, remember to open the flow and returns after the test :wink: