Generating a Negative Voltage
At least one of the converters must be galvanically isolated (input to output). There cannot be a common between input and output. Using two isolated converters can help you deal with noise and ground loops in an analog design. 
There is a potential issue with most single output converters not expecting a load that goes beyond their negative or positive rail. You can prevent any issues by putting a reverse-biased Schottky diode across each output. 
There is an advantage over using an inverting buck converter for the negative rail in that the startup surge of an inverting buck can be very high, enough to potentially pull down the input source and prevent proper starting.