There are two ways of increasing water pressure in you case, ether reduce the water flow (reduce number of outlets that demand water) or increase the main water supply flow rate. If you have a broken pump and you are drawing water from a source that is not designed to have more water drawn, then you will have a reduction in pressure (check on the water pump for max demand flow rate etc). If you don't have an isolation valve for your broken pump you can freeze the pipe and install one. Does the second system with the broken pump still retain a source of water that is not pressurized? it could be that by linking points of your working system into the broken system might cause a reduction in water pressure to compensate for pressurizing the second water supply. Overall i think its safe to say that there was probably a few tapped points off the working system at first, maybe to allow time for pump to be repaired, and poss. more have linked in without further investigation. Is the pressure low at every point in the system? Are all points in the system at the same ground level?
Dunk