Master Electrician Electric Contractor Richmond VA
- Henrico County, VA
- Electrical Contractor
All you're doing is proving that 1 + 1 = 1 - (-1).See graphic.
Regardless of labeling, the voltages must be in phase in order to add to twice one of them. This applies whether the source is two separate transformers or one with two secondaries.
However, with a single, center-tapped secondary, you can't invert anything; nor do you need to.