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I'm not aware of many consumer grade US sets with 2 HV transformers from back then. We did have 2 horizontal output tubes though on some sets.
Typical HV box on a US color sets was: the output tube, which could be any one of a variety of types, the flyback transformer - RCA and Zenith were different in design here, the HV rectifier, inevitably a 3A3 if it was an RCA based set, the focus rectifier - this got you your 5000 or so for focus - 2AV2 was one of the more popular ones here, 1V2 too, and the famous 6BK4 shunt regulator. Oh yeah and a damper of some sort. HV for focus was basically the kickback to the horizontal output tube's plate, the HV was stepped up in the normal way. RCA had the 3A3's plate plug directly into the transformer starting around CTC-15 or so. Lift the HV box cover and the tube pulls out with it. I'm guessing this was a safey feature....
The HV went to the tube and to the plate of the 6BK4, which was a fairly tall octal tube with a plate cap. The 6BK4 would keep the load on the HV system constant at all times, which of course kept the HV constant. I don't think the focus ever had regulation or even if it needed it....
By the end of the tube era, Sylvania and a few others used triplers that got rid of the 6BK4 and 3A3 and 2AV2, and of course their well known X ray issues.
Cathode current on most old US roundies based on RCA designs should be kept as low as possible, I think 220MA is the magic number where they start melting.
There were a few regulators besides the 6BK4 - I think GE made them, but all are beam triodes save for that bizzare one in the 50's. I think they all were compactron or octal based. By the time small portables came around, the pulse regulator method and other methods came into use, though I seem to recall RCA using 6BK4s on some sets right to the end, I suspect because they were really good at it or they just had a lot of them...
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