As stated by ErikdeBest, parallel tubes need half the plate load impedance, as well as double the current from the PS and they should have separate grid bias controls. I've seen some designs for parallel OP tubes that use a small resistor (100 ohm or so) connected in series in each plate circuit, to minimize current-hogging.
Drive signal voltage remains the same, of course, but drive current is doubled, since the OP tube grids are in parallel, grid resistance is halved and Miller capacitance is doubled. This is likely to be more of a concern if the OP tubes are triodes.