Doubling cone area and cutting power in half to each speaker (same power overall) still gains you 3db. Watts and excursion don't correlate at a 1:1 ratio, halving wattage to each speaker doesnt' drop excursion by half, so when you double cone are you gain displacement/output. You also sometimes gain thermal advantages, which can gain you more.
Doubling cone area AND power gains you 6db which is a doubling of total displacementl and acoustic power. 10db of loudness is generally considered where the sound "sounds" twice as loud. With bass it's slightly lower due to the hearing. This corresponds to about 10x the power keeping cone area equal. If you quadruple power and cone are doubles, that also gets you close to this mark. IE 1 speaker on 100 watts becomes 2 sepakers on 200 watts each. For bass this doesn't work as well due to the box size needed often messes up your cabin gain, which is the biggest overall contributor to bass in a car, which is why alot of car audio guys dismiss these rules. For mids and highs though they work fairly well. So for OP, if your amp can't drop impedence and you end up using 1/2 the power on 2x the cone area, your still going to get the 25% sound increase your looking for, imagine if your volume knob had 2 or 3 turns of clean volume left..