It is not that simple unfortunatley, first of all it is not good practice to only boost one side of the water, it is better to run a "balanced" supply from the header tank and then pump both so you have equal pressures at the outlet and you will not have any backflow or cross contamination issues providing it only supplies the bathrooms and not the kitchen sink witch has to be mains potable ( drinking) water and seperate from any other water. If you opt for pumping the hot only you will have to comply with regs as the cross contamination and back flow issues have to be strictly kept to and this needs building regs approval so only a registerd person can do this and certificate it. If you hook it up wrong without the correct proceedures and fittings you could end up contaminating you're drinking water and the authorities have very heavy fines and possible manslaughter charges for any fatalities caused through cross contamination. You also may need to upgrade the header tank as it may not be able to deliver the larger volumes of water required at the rate you're pumping the water.