polygon tool in Eagle PCB
The planes have a so called rank setting.

Planes with a higher rank (like in sports, 1 is ranked higher than 2) will take priority and be filled first. Lower rank planes will automatically cut themselves out around the higher ranked planes. The clearance will be the larger of the two isolate settings (if one plane has an isolate of 12mil, and the other 24mil, a 24mil gap will result).