FAS2240, FAS2552, FAS2750 models are single chassis HA pair , if you have two controllers (two in a box).Both the controllers shares the same PSU.
Each PSU is capable of powering on the whole chassis (not node specific) which means both the nodes can run with one PSU.
If the other PSU also stops working then the complete chassis will loose the power..... in two node models FAS with two power supplies.