[math-fun] time dilation

in http://www.nist.gov/pml/div688/2013_1_17_newera_atomicclocks_3.cfm a atomic clock, using Y-Sr ‘lattice’, is said to achieve 3E-18 accuracy & stability. This would allow to detect time dilation caused by a mere 4 cm altitude difference. If guestimating correctly, this would be equivalent to the gravitational effect of a 19 kg mass at a horizontal distance of 10 cm. Question: would a second 19 kg mass on the opposite side anihilate the effect (null net acceleration) or double it ? Is gravity a normal ‘garden&kitchen’ vectorial field ? Wouter.