EnergyPlus example file 5ZoneWLHPPlantLoopTower.idf models similar configuration with boiler and tower in parallel configuration. SetpointManager:Scheduled:DualSetpoint can be used at plant supply outlet node (i.e. after mixer) to specify high/low temperature setpoints. Plant Loop Demand Calculation Scheme should be set to DualSetpointDeadband. Engineering reference has more information on working of DualSetpointDeadBand scheme.