> Does anyone know if the method for plotting Maximum Plated Dissipation is the same?
In general, two plates is twice the heat-throwing area and can throw twice the heat.
In small-signal stages, it rarely matters. Take the mature Fender 12AX7 stage: 100K plate resistor, 1.5K cathode resistor, running say 333V supply: it sits at 1mA and about 231V across plate to cathode, 0.231W dissipation, rating is 1.0W. We could run over 600V supply and be inside dissipation rating. But this would give over 400V on the tube, past the plate voltage rating. In fact you can't hardly cook a 12AX7 if you try.
If you double the tubes and double the fixed conductances (halve the resistor values, 50K and 750R), it is still the same. If one tube didn't cook, two tubes won't.
If you double the tubes and stick with 100K 1.5K resistors, you have double the plate area to throw-off roughly the same heat. Actually with two 12AX7 and 100K and 1.5K resistors, the current will increase and the tube voltage will decrease. There may be some case where tube heat would increase, but I doubt it, not on any happy amplifier stage.
So a known-good one-triode plan, doubled-up, with either same or half resistor values, will be safe for heat.
You should ponder current-sharing. At same electrode voltages, two units can be as much as 40% apart in curent. So if you design for 100% Pd, one unity could be 80% and the other could be 120%. However we almost never see this much difference, especially in twin-triodes. If you are really working near 80% Pd, separate cathode resistors will reduce the difference. As you get past 90% you should test for current-hogging (or just be less reckless).
Some of the bigger twin-triodes, e.g. 6CG7, do have limits like 3.5W/plate but 5W total both plates. You can run one plate at 3.5W if the other is much less; if you push both then they are 2.5W max each.
I plotted 6CG7 and the only way I could find Pd trouble is with 500V supply and 18K load. Run 2.5K bias, you land near 250V and 14mA or 3.47W. One of those is fine; two of them (9K and 1.25K) in one bottle exceeds 6CG7 rating. But 500V and 18K load is a VERY untraditional affair, moreso for a 9K load. And raising that to 13K puts us back inside the 5W/bottle rating. And if you really need 500V and ~~10K, there are other (heftier) choices, like the TV V-sweep tubes, which today are often cheaper than the hi-fi fad preamp bottles.
If you are doubling BIG tubes, simple: one 6L6 at 350V and 5K will make 10W, two 6L6 at 2.5K will make 20W. (However once you spend for two tubes, your output transformer costs can be much less if you go push-pull rather than simple parallel.)