I assume that the carb is not part of the tank,does not bolt directly to it, if that is the case, get a metal 5 gal gas can, we then soildered a shut off in the can ,in the hole we drilled at the bottom side, gas line to the card make can higher than motor-- a 5hp briggs, with a transfer pump will run 20-23 hours on a fill, if the card is bolted to the tank, I do not think it can be done?
If I "T" into the existing fuel line.
Would I need a [censored] off valve at the "T"?
It shouldn't mater if the two tanks are open to the carb at the same time, but what if one tank was empty?
Would this cause a vacum and pull air into the line?
If you tee into the existing fuel line you would feed from both tanks if they are both the same height.If one was higher it would fill the other tank.Not a problem unless the lower one is vented.Probably would be better off to convert to propane and run a larger propane tank.Probably cheaper to fuel also.