815 dual tetrode
Two power tube systems within one bulb are somewhat uncommon. This type from RCA incorporates two beam-power tetrodes with 10 W plate dissipation each. Plate connections are to the two top caps. Screen grids of both systems are connected together to one base pin, as well as both cathodes and beam-forming electrodes. Heaters are center-tapped, so the tube can be heated with 6.3 V, 1.6 A or 12.6 V, 0.8 A respectively. This tube has a real huge octal base - compare with the socket I have plugged it in!
 
 
It was sometimes used in RF power oscillators. And - as I have seen during a stay in the former Soviet Union - widely in cinema and public sound systems there in the final push-pull stage. I don't remember, if it was just this type, but a very similar one at least, also with the two top caps. Operated in Class-B service, I assume, such an "integrated" one-tube push-pull final stage was able to provide up to about 30 W of audio power to a speaker box. And they had whole racks there full with these amps!