Using voltage divider vs using a DC-DC converter
Are you asking how to regulate 24V to 18V? What kind of current do you need? The easiest way to get a few dozen-to-hundred mA would be to use a linear regulator.
A voltage divider will consume a TON of power and murder your efficiency. And its output will vary depending on the load. IMO, it's really only an option if you're looking to get a reference voltage, driving a very high impedance load
I wouldn't use multiple DCDC converters. Its wasteful and inefficient (why spend dollars when you can spend a few cents?). I'd source a single buck converter with the specs I need, or use a linear regulator if I only needed a few watts of output