WARNING: Tube/Valve amplifiers use potentially LETHAL HIGH VOLTAGES.
Building, troubleshooting and testing of these amplifiers should only be
performed by someone who is thoroughly familiar with
the safety precautions around high voltages.
As far as the current calculation is concerned, you could wire them in series. There is a possible concern that they might not divide the voltage equally. So connecting one of the tubes to one 6.3V winding, and the other to the other winding, would be better. But the basic answer is yes, the transformer could handle the load.