Switching between 2 24V 30A power sources
You can use MOSFETs, but you must gang two of them source-to-source in each power path so that the body diodes prevent current backflow down the 'inactive' path. 

One such example (by no means unique) is the LTC4418, a dual channel prioritized power-path controller. A sample application is shown below: 



(Image courtesy of Analog Devices)

The voltage thresholds are set with resistors (which includes some level of hysteresis) and the chip then drives the gates of the respective power path, providing a resistance that is 2*RDSon for each path. You can also parallel MOSFETs to lower this further -- MOSFET selection is dependent on parameters such as form factor, SOA, gate charge, allowable operating temperatures, etc., so this is left as an exercise for the reader.