The valve that supplies water to those two heads is probably stuck open. It may be something lodged between the diaphragm and seat, or maybe a worn or pitted diaphragm seat. A tiny leak at the valve diaphragm will cause water to dribble from the top of a head, usually most notable at the lowest head on the line. Easy to fix if your handy. Remove top portion of the valve and check for obstructions, and or a pitted diaphragm seat. Be carefull during disassemble as there is a spring in there that should not be lost. Also do not forget to shut the water off first.