With the looks of the setup that
@smith716 did it looks like you only fill the new tank which also flows gas into the original tank as if they were simply a split tank setup like on the ST1300. No need to flip a switch or drain 2 tanks, other than the volume of gas that is in both tanks. Only question is if the fuel system pressurization is right to avoid future damage to the fuel system and injectors.
On another note... I'd hate to loose the storage that kind of setup takes under the seat. Why I'd prefer to stay with a bottle of spare gas, maybe adding a tool tube hanging on the outside someplace like under a luggage rack or top case that would not take up all the precious storage space for other things I carry.