Looking to brew and steam simultaneously? If so, you're going to need a heat exchanger espresso machine! Heat exchangers transfer water from reservoir through the boiler via a tube designed to heat the passing water so it reaches optimal brew temperatures. It also permits your espresso machine to brew and steam at the same time!
Heat Exchanger Espresso Machine vs. Single Boiler
With a single boiler you're not able to brew and steam simultaneously. It's recommend to steam first, then burn on a single boiler espresso machine.
Heat Exchanger Espresso Machine vs. Dual Boiler
Dual boilers contain two boilers - each responsible for one single function (either brewing or steaming). Dual boilers increase efficiency and consistency by reducing the amount of pressure lost during use, especially if brewing and steaming simultaneously.