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.
2X "6" V. tubes of the same type and same production run, with their heaters in series, will be fine connected to the toroid. You could bias the junction point of the 2 heaters off B+, for noise control.
Take the center connection of the two heaters, the two that you connected together and are not connected to the transformer, and place about 50V of bias using a resistor divider from B+. It's not a requirement, but can reduce hum.
In addition to biasing that point off B+ with a resistive voltage divider, you ground that point via an electrolytic cap. of several μF. As the other posters stated, the network of 2 resistors and 1 cap. reduces vulnerability to hum and other noise.