DOES NOT ACCOUNT FOR POWER COMPRESSION
SE12 Reference Efficiency = 85.2 dB (1W/1M) ... Power handling = 600 watts RMS
Sealed ...
For one sub with 600 watts to it ... 85.2 + 10*Log(600) = 113 dB
For two subs with 300 watts to each ... 85.2 + 10*Log(600) + 10*Log(2) = 116 dB (+3 dB gain)
If you were to send 600 watts to each ... 85.2 + 10*Log(1200) + 10*Log(2) = 119 dB (+6 dB gain)
By porting a single sub, you theoretically gain up to 6 dB near the tuning frequency, but it depends on many factors ...
So, short answer, sealed should be a louder, but not by much ... Not to mention ported is much more cost effective when you consider the difference between a 600 and 1200 watt amp, another sub, upgrades to your electrical system to handle the extra power, etc. etc. etc.