Q: How can i schedule the boiler to work just when the outdoor drybulb temperature is below 5°C, and the heat pump when it's above 5°C.
A: Create two schedules based on the temperatures in the weather file. You can do this by reporting Outdoor Dry Bulb hourly, then make a spreadsheet with two columns, one which =1 whenever ODB>=5, and the other which =1 whenever ODB < 5. Save this spreadsheet as a csv format file, and then you can use Schedule:File to read these as EnergyPlus schedules. Use these schedules in the PlantEquipmentOperationSchemes object to make "boiler heating" active in cold weather and "heatpump Heating" active in warmer weather.
Note that you will need to have two PlantEquipmentList objects, one which lists only the boiler, and the other which lists only the heat pump. And the two different PlantEquipmentOperation:HeatingLoad objects should reference different PlantEquipmentList objects.
I must admit that I am not 100% certain this will work as you want. I would suggest that you report temperatures and flow rates at selected points on the hot water loop to see if things are working properly.