If you will double the voltage, then wire this two HT- windings in "serial".
The "parallel" option can not be realized with a CT- HT winding
![Seeing Stars :stars:](./images/smilies/seeingstars.gif)
The use of a CT- HT (for a two way silicon diode rectification) is simply ancient and has no advantage in comparsion to a "modern" bridge rectification.
BTW: I have absolutely no clue why so many US amp builders still use a CT- Powersupply nowadays
![Confusion :what:](./images/smilies/confused.gif)
The CT for the heater winding is also obsolete, IMO.
A "virtual" CT via 2x 100R or (even better) a good quality 100R Pot is safer. Plus: less manufacturing costs
![Thumbs Up :thumbsup:](./images/smilies/happy0159.gif)
Regards
Robert