When a build is done on a Sprinter, the stock sending unit comes with one auxiliary line that can be tapped into in order to get diesel out of the tank to run a heater or something else on the Sprinter. If they have two devices that need diesel to run on the Sprinter, they will add an aftermarket Auxiliary Fill Cap shown in the picture below.
While the this type of Auxiliary Fill Cap can be put on the S&B tank, we recommend to T-off of the 1 aux fuel tap coming from the sending unit when you have two heaters, so that you don't drill into the S&B tank as this can result in a leak. If you do use the aftermarket fill tap, you need to make sure to put it high enough that it doesn't leak when the tank is full or sloshes and that the tank will still fit with it installed. S&B is not responsible for any leaks related to the installation of an Auxiliary Fill Cap.