The output of each turbine will first have to be regulated so each turbine gives the same voltage for a given rotational speed distribution. These regulated output voltages can then be combined via identical power diodes.
If any one turbine drops below the power threshold rotational speed (i.e., the regulated output drops below what it should be regulated to) then that turbine will stop providing power to the main combined output.
Due to the fact that no two diodes have exactly the same forward voltage drop (unless you pay through the nose, ear, mouth, and any other bodily orifices that you may have), each turbine's regulated output should be adjusted so that the output voltage after its coupling diode is equal to all the others. A slight difference in this output voltage could cause the diode to become reverse biased (if it is too low) and never provide current to the main output unless all the other turbines are stopped.