The LARGE Silvertone 6-10" amp with four 6L6's may well have had two output transformers, as it was almost wired as a two-channel stereo amp...but not
@Default
As Rambozo said, the 4x6L6 version used two output transformers, one for each pair, identical to the one in the 2x6L6 version. I'm guessing Nat Daniel got a better price for the one in quantity.
A 'handful' (at least one) of the prototype Marshall 100W amps used two output transformers too. I'm guessing it was a proof-of-concept thing for them before ordering custom transformers.