You want two boilers, and regulated, easily-modulated temps at the group.
PID'd SB/DU machines (Silvia, Gaggia, Solis) are sufficient for home users.
Pull a shot, flip the steam switch; get the milk, pour it into the pitcher, and by that time the boiler is at steaming temp.
If you want to pull another shot right away, pull fresh water through the steam wand until the boiler temp has dropped to ~195°F, and let it set for a few seconds for the PID to stabilize the brew temp.
While waiting for the brew temp to stabilize, grind and tamp the coffee for the next shot.
Double boiler are a convenience, not a necessity for home espresso machines.