WARNING: Tube/Valve amplifiers use potentially LETHAL HIGH VOLTAGES.
Building, troubleshooting and testing of these amplifiers should only be
performed by someone who is thoroughly familiar with
the safety precautions around high voltages.
A common application for double triodes is as a differential amplifier such as the driver stage in a Williamson amp, or as a long tail pair phase splitter such as in Mullard, Leak etc. Two equally specified triodes in one envelope tend to be better matched than two completely separate tubes. There are a few types, though, where the two triodes are deliberately not equally specified and are meant to serve different purposes. Putting two triodes in one envelope saves not only space but also cost, which is important for manufacturers.