Syphoning fuel.

[SimT]

I have 2 Sigg tanks in parallel with a vent for each.
When I fill the tanks, one fills first and I put a finger over the vent of the first till the second fills.
When both tanks are brim full the fuel syphons out of the last tank to fill and leaves a puddle of fuel till that tank empties out onto the floor down to the bottom of the vent nipple inside the tank. I have been using a pipe to blow back to try and avoid syphoning but not a satisfactory solution.
I could stop refuelling when the first vent starts to overflow but how can I be sure both tanks are full unless I see both overflow?

[Skiddz]

Your vent line needs to terminate above the vent nipple(s) high enough to stop the siphoning.

Mine "T" into one line that exits just below the boom and other than a couple drops as a result of filling the tanks, nothing: leaks.

[SimT]

Just put the vent pipes up under the boom and it works a treat.
Not as elegant a position as the vent pipes exiting under the baseplate but at least fuel is not getting everywhere.
Thanks.

Simon