Another subwoofer question

Which would have more spl? one 12 inch sub ( we will say brand A that's rated for 1200 watts rms ) in a ported box built to spec with a amp delivering 1200 watts VS 2 12 inch subs ( this would be brand B where each sub is only rated for 600 watts rms ) in a ported box built to spec with 600 watts going to each sub?

Depends on their efficieny along with box design.

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Just based off what you said. The 2 12's would be louder more cone area for boat enviorment.

Power handling has zero correlation to output.
As a matter of fact, a sensitivity/efficiency rating has very little bearing on maximum output with the way woofers are designed today. The rating is typically taken at 1 watt which tells you next to nothing about how the woofer performs at 50 Hz with 500 watts.
You cannot compare unqualified specs from brand to brand.

Here's the answer....
If both are executed correctly then it's almost a certainty that two 12"s with 1200 total watts would play louder than a single twelve with 1200 total watts. It's more efficient to create output with more surface area than more excursion.

Another point which is worth going into some detail on is the point that Cowwboy made, "depends on....box design."
We can only answer the question based on the way it is posed and assuming that all execution for both scenarios is exactly the same in, for example, orientation, box loading type, location, and so on.
If the two woofers are separated then they may lose the advantage.
If one scenario is direct radiating and the other scenario is concealed in a console or locker then that changes everything.