Oh yeah my guess was correct then- two 12v batteries in series then 0V is -12V, connected +12V point is 0V and 24V is +12V. It is just about 12V up and down potential difference, not exactly Negative voltage needed. I understand!
In 12-0-12 transformer (after around 14.5-0-14.5 smooth DC output), should I have to connect 'LM7812' in series of Positive line to get +12V and in another- should I have to use 'LM7912' in series of Negative line to get -12V? I think Yes.