[Excaliber]

Paralell is:
Negative terminal to negative terminal.
Posit terminal to posit terminal.

So yes, "splice" the posits and negets together using jumpers or whatever good method you can.

Series is Posit to Neget criss cross. Get it right and you get 24 volts, get it wrong and you blow up the batteries and go "Oops".

I don't think "paralell" (still 12 volts) will increase or double the cold cranking amps output BUT it will sustain that output longer. Which is really what the CCA measurement is trying to tell you anyway. How LONG can I crank my engine before the battery goes dead sort of thing. That time is "doubled". Your current single battery CCA should be plenty to start even a mean motor!