  We investigate a two-parameter entropy introduced by Schw\"{a}mmle and
Tsallis and obtain its probability distribution in the canonical ensemble. The
probability distribution is given in terms of the Lambert W-function which has
been used in many branches of physics, especially in fractal structures. Also,
extensivity of $S_{q,q'}$ is discussed and a relationship is found to exist
between the probabilities of a composite system and its subsystems so that the
two-parameter entropy, $S_{q,q'}$, is extensive.
