Jack & Jill Heath together form the Heaths, meaning the Heath family. If you are referring to something they own jointly, they you'd write the Heaths' house, car, yard...
'Heath' has a couple of distinct senses: 1: a tract of level wasteland; uncultivated land with sandy soil and scrubby vegetation; 2: a low evergreen shrub of the family Ericaceae; has small...