It's a slightly special arrangement, closest to PSE (parallel single-ended) in terms of how it sounds. We usually call it "bridged" which is not precise but helps make it clear this is not the usual PSE connection. You could think of it as tubes in parallel but transformers in series.
The problem with simply paralleling triodes is that the tube internal resistance is much smaller than the load resistance. So any difference between the tubes causes difference-signal current through the other tube. Sometimes this is called "current hogging." We, and many others, have noticed some degradation of sound quality with PSE amps.
My solution is to run the individual amps with the same input (so they are in phase with each other), but wire the secondaries of the transformers in series. This greatly reduces the interaction between the tubes, while leaving intact the single-ended harmonic structure. It doubles the design speaker impedance, so I designed the output transformers in Kaiju to have an additional tap at 2 ohms so that the series connection can be made for a 4 ohm load. The usual 4-8-16 ohm taps can be used to handle 8, 16 or 32 ohm loads in this bridged mode.
For some years, the large and complex Bottlehead reference system used four 300B amps per channel wired this way to drive the midrange speakers, which gave us a good deal of experience with this connection.
Incidentally, this is not an issue when paralleling pentodes, which have high internal resistance. I have designed, and we have built, an experimental pair of SE parallel pentode monoblocks with 6550s, making over 50 watts each. These amps us a single plate choke and output transformer each.
