John,
Prior to moving into the world of commercial lever machines I was really close to pulling the trigger on that 2-group Ponte Vecchio Lusso. At the time, OrphanEspresso was selling slightly modified Lussos, so I had the chance to speak with Doug and Barb about them. They said that one of the reasons Ponte Vecchio went that route was to allow the user to pull more shots without overheating, plus it just looks cool! Now, I never got to experience that, as I never bought one, but that same thought was the main reason I was looking at the 2 group and not the one group. The reason I started looking at new machines was that the MCaL would overheat after 3 shots, which was really inconvenient when company was over. With that said, understand that home lever machines are not meant to be left on all day. If you leave those machines on long enough, regardless of how many groups the machine has, the entire machine will heat up to an incredibly hot temp. So, just realize you turn them on when you want shots and turn them off when you do not. They do heat up relatively fast, so it is not really an inconvenience.
The other plus to a Lusso versus an Export is the water capacity, 3L to 1L. Regardless of how many shots I could pull on the MCaL, utilizing tricks to cool it or not, the machines water capacity, 1L, really only lent itself to about 5-6 doubles with flushing in between shots. There may have been more water in the boiler, but with Elektras they recommend you always keep the boiler right around half. This ensures you are not exposing the top of the heating element.
As far as two back to back shots on the Export, I will say not immediately back to back anyway. Lever espresso machines do not have a group pressure release, so the user has to wait for the pressure to dissipate. Otherwise they get the feared / messy portafilter sneeze. Even if you cheat and wiggle the portafilter to help it along with it's pressure release you should still wait 25-30 seconds to let some of the pressure subside. After which you could wiggle the PF to remove the rest, then flush the group to remove the mess you made on the group screen due to your lack of patience, dry the screen off with a towel and then you could reload for another shot.