What is the difference between a heat exchanger and dual boiler espresso machine?

A heat exchanger (HX) machine has one boiler for steam, and a copper tube running through it heats water for brewing — allowing simultaneous steaming and brewing but with less precise brew temperature control. A dual boiler has separate boilers for steam and brewing, offering independent temperature control for both. Dual boilers are more expensive but produce more consistent shot temperatures, making them preferred by serious home baristas.