If this were a cathode-driven amplifier, the drive for two tubes wouldn't quite be double. Putting two tubes in parallel doubles the transconductance. Twice the anode current for the same grid voltage. More power from the exact same drive voltage.
Since the RF drive power is pushing current into the cathode circuit, you'll need a little more drive for two tubes than for one, but not nearly double.
Grid drive is almost entirely about voltage. Only tiny bit of the RF drive current reaches the tubes, 10 percent or less. As such, the exact same RF drive voltage will drive two tubes the same as it drives one. But this is only strictly true for class AB1, where the drive level is held below the threshold where the grid draws current.
Class AB2 is more the rule, especially for AM. I would predict that two tubes require about ten percent more drive than one tube using grid drive.
Mind you this is an educated guess, in light of the fact that the tubes absorb at most ten percent of the RF drive power. The other ninety percent warms up the swamping resistors.
Of course, the unwanted positive feedback in the grid circuit also contributes to warming the swamping resistors. Not a lot, but that's what they do, is absorb that regenerative feedback so it can't make the tube oscillate.
73
