I understand the reasoning behind the E61. This whole double boiler thing puzzles me.
It seems most commercial machines ansswer the steam problem with much bigger boilers instead. I don't forsee me beeing efficient enough to really worry abouta 30 sec delay between the shot and the steam.
Most commercial machines use heat exchangers (HX). These typically have large boilers because the water in the HX is flash heated by the boiler water as fresh/cool water is passed through the HX on the way to the group. As an added benefit these large boilers also make prodigious amounts of steam.
HX: Designed for commercial operations where they seldom sit idle. When they do sit idle the temps of the group & HX tube/water will rise to stabilize at boiler temp (usually ~253°F). Way too hot for making espresso, it's necessary to flush fresh water through the HX & group to bring the temps down into the brewing range (195° - 203°).
see note 1Double Boiler: Each boiler is kept at the temp best suited for its purpose. The brew boiler is always at the temp selected that's within the range for brewing and the steam boiler is always at a temp which provides lots of steam power.
Single Boiler/Dual Use: Uses the same boiler for brewing and steaming. After a shot is pulled a switch is flipped and a circuit is activated which will bring the boiler temp to steaming temp (~300°F). Because of the temp changes it take a while to go from brew temp to steam temp and then back to brew temp.
edited - note
1: Brew temps in an HX are controlled by a combination of boiler temps which flash heat the water in the HX, how long the water stays in the HX tube as the shot is pulled, and the temp of the group. If permitted to sit idle, the HX & group will over heat and must be cooled down to brew temp before the shot is pulled.