Fluid will find level, so the extra tank would need to be level with the tank mounted on the gen set.
If the extra tank is level with the mounted one & both filler caps are of the same height, you could run the hose from the extra tank, into a T in the hose coming off of the mounted tank & gas would flow from both tanks.
If the extra tank was to be lower than the tank mounted on the gen set, if the tank mounted on the gen set had the filler cap sealed, then once fuel had stated to be used from the mounted tank, a vacumm would be created & the fuel from the extra tank be pulled into the on gen set mounted tank. The extra tank couldnt be very much lower than the on gen set mounted tank, because that vacumm will not be very strong to pull the fuel from the extra tank.
Neil
