I agree with Bill. And I'm surprised you switched to 2x14mm before figuring out which was more powerful.
I guess it's not simple but I think we used to estimate it based on the rubber(s) cross-section area ( subtracting the usual big or small hole in the middle). But other factors come into play, such as length and stretch and rubber compound.
I too would expect 2x14mm to be more powerful. That said, I currently prefer a single 16mm bulk rubber. I've tried 20mm bulk, and bulk and screw-in 18mm bands ( I quite liked 18mm screw-in).
I also have a little used little 65cm speargun, 2x14mm. Which is easier to load than a thicker band but more time-consuming than a single band. The single 16mm bulk rubber seems like a very good compromise for the British waters I fish in (with a 75cm-90cm barrel length).
If visibility allows, as Bill already suggested, a longer speargun might be a better choice. It might well give better power, "balance" and "pointability" (I feel more accurate with a longer speargun when targeting fish that aren't close).
BTW I often take a 75cm and a 90cm Omer XXV speargun out with me, for the variable British visibility. Although I often think a single 82cm Omer XXV ( as recommended by wise and venerable former forum members: OMD, Spaghetti and Pavel) would do the same job more simply. [I think the 82cm model was not available to me at the time though :/ ].
If visibility is very good, it might be worth considering longer still e.g. 95cm, 100cm, 105cm or 110cm. But they will likely be harder to load, depending on bands used.