Availability managers in plant or air loop will turn on or off the entire loop (not individual dry or evaporative fluid coolers) so they may not serve the purpose in this case. If the dry and evaporative fluid coolers are in parallel branches with their branch pumps, these pumps' schedule can be modified using EMS. (Something like below). You can try and see if this works.
Another lot more time consuming but accurate way could be to use PlantComponent:UserDefined. This object will allow to define the behavior of hybrid cooling tower.
 Branch,
  Condenser Supply Fluidcooler Branch 1,  !- Name
  0,            !- Maximum Flow Rate {m3/s}
  ,             !- Pressure Drop Curve Name
  Pump:VariableSpeed,    !- Component 1 Object Type
  Cond Circ Pump 1,      !- Component 1 Name
  Cond Circ Pump 1 Inlet Node,  !- Component 1 Inlet Node Name
  Condenser Fluidcooler Inlet Node,  !- Component 1 Outlet Node Name
  ACTIVE,          !- Component 1 Branch Control Type
  Fluidcooler:SingleSpeed, !- Component 2 Object Type
  Big FluidCooler,     !- Component 2 Name
  Condenser Fluidcooler Inlet Node,  !- Component 2 Inlet Node Name
  Condenser Fluidcooler Outlet Node,  !- Component 2 Outlet Node Name
  ACTIVE;          !- Component 2 Branch Control Type
 Branch,
  Condenser Supply Fluidcooler Branch 2,  !- Name
  0,            !- Maximum Flow Rate {m3/s}
  ,             !- Pressure Drop Curve Name
  Pump:VariableSpeed,    !- Component 1 Object Type
  Cond Circ Pump 2,      !- Component 1 Name
  Cond Circ Pump 2 Inlet Node,  !- Component 1 Inlet Node Name
  Condenser EvapFluidcooler Inlet Node,  !- Component 1 Outlet Node Name
  ACTIVE,          !- Component 1 Branch Control Type
  EvaporativeFluidCooler:SingleSpeed, !- Component 2 Object Type
  Big EvaporativeFluidCooler,     !- Component 2 Name
  Condenser EvapFluidcooler Inlet Node,  !- Component 2 Inlet Node Name
  Condenser EvapFluidcooler Outlet Node,  !- Component 2 Outlet Node Name
  ACTIVE;          !- Component 2 Branch Control Type
 Pump:VariableSpeed,
  Cond Circ Pump 1,      !- Name
  Cond Circ Pump 1 Inlet Node,  !- Inlet Node Name
  Condenser Fluidcooler Inlet Node,  !- Outlet Node Name
  0.001388,         !- Rated Flow Rate {m3/s}
  297500,          !- Rated Pump Head {Pa}
  500,           !- Rated Power Consumption {W}
  .87,           !- Motor Efficiency
  0.0,           !- Fraction of Motor Inefficiencies to Fluid Stream
  0,            !- Coefficient 1 of the Part Load Performance Curve
  1,            !- Coefficient 2 of the Part Load Performance Curve
  0,            !- Coefficient 3 of the Part Load Performance Curve
  0,            !- Coefficient 4 of the Part Load Performance Curve
  0,            !- Minimum Flow Rate {m3/s}
  INTERMITTENT,       !- Pump Control Type
  CondCircPump1PumpAvailSched;  !- Pump Flow Rate Schedule Name
 Fluidcooler:SingleSpeed,
  Big FluidCooler,     !- Name
  Condenser Fluidcooler Inlet Node,  !- Water Inlet Node Name
  Condenser Fluidcooler Outlet Node,  !- Water Outlet Node Name
  NominalCapacity,     !- Performance Input Method
  ,             !- Design Air Flow Rate U-factor Times Area Value {W/K}
  58601.,          !- Nominal Capacity {W}
  51.67,          !- Design Entering Water Temperature {C}
  35,            !- Design Entering Air Temperature {C}
  25.6,           !- Design Entering Air Wetbulb Temperature {C}
  0.001388,         !- Design Water Flow Rate {m3/s}
  9.911,          !- Design Air Flow Rate {m3/s}
  autosize;         !- Design Air Flow Rate Fan Power {W}
 Pump:VariableSpeed,
  Cond Circ Pump 2,      !- Name
  Cond Circ Pump 2 Inlet Node,  !- Inlet Node Name
  Condenser EvapFluidcooler Inlet Node,  !- Outlet Node Name
  0.001388,         !- Rated Flow Rate {m3/s}
  297500,          !- Rated Pump Head {Pa}
  500,           !- Rated Power Consumption {W}
  .87,           !- Motor Efficiency
  0.0,           !- Fraction of Motor Inefficiencies to Fluid Stream
  0,            !- Coefficient 1 of the Part Load Performance Curve
  1,            !- Coefficient 2 of the Part Load Performance Curve
  0,            !- Coefficient 3 of the Part Load Performance Curve
  0,            !- Coefficient 4 of the Part Load Performance Curve
  0,            !- Minimum Flow Rate {m3/s}
  INTERMITTENT,       !- Pump Control Type
  CondCircPump2PumpAvailSched;  !- Pump Flow Rate Schedule Name
 EvaporativeFluidcooler:SingleSpeed,
  Big EvaporativeFluidCooler,  !- Name
  Condenser EvapFluidcooler Inlet Node,  !- Water Inlet Node Name
  Condenser EvapFluidcooler Outlet Node,  !- Water Outlet Node Name
  3.02,           !- Design Air Flow Rate {m3/s}
  2250,           !- Design Air Flow Rate Fan Power {W}
  0.002208,         !- Design Spray Water Flow Rate {m3/s}
  UserSpecifiedDesignCapacity,  !- Performance Input Method
  ,             !- Outdoor Air Inlet Node Name
  ,             !- Heat Rejection Capacity and Nominal Capacity Sizing Ratio
  ,             !- Standard Design Capacity {W}
  ,             !- Design Air Flow Rate U-factor Times Area Value {W/K}
  0.001703,         !- Design Water Flow Rate {m3/s}
  87921,          !- User Specified Design Capacity {W}
  46.11,          !- Design Entering Water Temperature {C}
  35,            !- Design Entering Air Temperature {C}
  25.6;           !- Design Entering Air Wet-bulb Temperature {C}
 Schedule:Compact,
  CondCircPump1PumpAvailSched,  !- Name
  Fraction,         !- Schedule Type Limits Name
  Through: 12/31,      !- Field 1
  For: AllDays,       !- Field 2
  Until: 24:00,0.0;     !- Field 3
 Schedule:Compact,
  CondCircPump2PumpAvailSched,  !- Name
  Fraction,         !- Schedule Type Limits Name
  Through: 12/31,      !- Field 1
  For: AllDays,       !- Field 2
  Until: 24:00,0.0;     !- Field 3
 EnergyManagementSystem:Actuator,
  Set_CondCircPump1Avail_Sched, !- Name
  CondCircPump1PumpAvailSched, !- Actuated Component Unique Name
  Schedule:Compact,       !- Actuated Component Type
  Schedule Value;        !- Actuated Component Control Type
 EnergyManagementSystem:Actuator,
  Set_CondCircPump2Avail_Sched, !- Name
  CondCircPump2PumpAvailSched, !- Actuated Component Unique Name
  Schedule:Compact,       !- Actuated Component Type
  Schedule Value;        !- Actuated Component Control Type
 EnergyManagementSystem:ProgramCallingManager,
  My Setpoint Schedule Calculator Example,
  BeginTimestepBeforePredictor,
  Set_CondCircPumpAvail_Sched_Prog;
 EnergyManagementSystem:Sensor,
  OutdoorTemp,              !- Name
  Environment,              !- Output:Variable Index Key Name
  Site Outdoor Air Drybulb Temperature;  !- Output:Variable Name
 EnergyManagementSystem:Program,
Set_CondCircPumpAvail_Sched_Prog,
IF (OutdoorTemp <= 18.33 && OutdoorTemp > 0.0),
 Set Set_CondCircPump1Avail_Sched = 1.0 ,
 Set Set_CondCircPump2Avail_Sched = 0.0 ,
ELSEIF (OutdoorTemp > 21.0),
 Set Set_CondCircPump1Avail_Sched = 0.0 ,
 Set Set_CondCircPump2Avail_Sched = 1.0 ,
ENDIF;
 Output:Variable,CondCircPump1PumpAvailSched,Schedule Value,timestep;
 Output:Variable,CondCircPump2PumpAvailSched,Schedule Value,timestep;
 Output:Variable,Cond Circ Pump 1,Pump Electric Power,timestep;
 Output:Variable,Cond Circ Pump 1,Pump Mass Flow Rate,timestep;
 Output:Variable,Cond Circ Pump 2,Pump Electric Power,timestep;
 Output:Variable,Cond Circ Pump 2,Pump Mass Flow Rate,timestep;