Low Voltage DC Automatic Transfer Switch
If each input is 12V then they can be tied together with schottky diodes to form a common input. Should one supply fail the other will still provide power to the output. The net voltage after the diodes will be slightly less than 12V - probably about 11.5V and if this is critical a boost converter can be used afterwards to restore 12V. The boost converter, if designed correctly could also take inputs that are from 5V to 20V and still provide a 12V output. This would be a buck-boost switching regulator.

However, if you can live with 11.5V, just use two schottky diodes rated at probably 5A each.