Yes, the solenoid fuel valve would be powered (open) only when the key is on and the fuel switch is turned on. This way any setting of the petcocks and the fuel switch the fuel flow will be blocked with the key off. If I can fit this in the space available the petcocks could be left open until tank removal for servicing is required.
The instructions I am referring to are the installation instructions that can be downloaded from Touratech's fuel tank description. The way I see the relative positions of the bike's under seat tank and the TT tank you don't want the fuel lines open when filling both tanks.
Perhaps after you have some miles on the tank you can offer a review on what is working and anything that we can improve. I will be ordering next month and depending on order status I should get the project completed a couple weeks later. In addition to the tank I will be changing springs and redoing my accessory wiring harness so I don't have any fuses or junction boxes under the new tank. It will be fun