The Big Muff Pi and its baby brother, the Little Muff, were, and remain, different beasts from the Muff Fuzz and Double Muff, even though there were historically different versions of each.
Just as there was an op-amp Big Muff, there was also an op-amp Muff Fuzz. However, the "classic" Muff Fuzz was essentially a silicon Fuzz Face structure, with a fixed gain, and a pair of clipping diodes on the output to give a little more edge. The Double Muff simply sticks two in series, with a switch to back down to only one. Both subcircuits have the same gain and structure, except that the first one in seriesx shaves off a teensy bit of top end to compensate for the fact it will be reclipped. When in series, the output level of the first one serves as a sort of drive-sensitivity for the second one.