Car audio Smps transformers in series

What would be the proper way to series the secondaries of transformers in basic two transformer based amplifiers?
I've done it successfully except struggling figuring out grounding the secondaries at this point.Example being for the scope ground.
I'm assuming I have no phasing issues seeing that the voltage essentially doubled by connecting the secondaries.
Could someone give me the necessary guidelines to connecting these style 2 primary 2 secondary windings style transformers in series correctly and how ground of the secondary is handled.
Still wondering if the firing order for the 4 primary windings is crucial with the secondaries series'd.