would having 2 enclosures, a ported playing something like above 25hz, and a sealed playing very low bass notes (like 25hz or under i'm thinking) sound bad? i like the low bass rumble of a sealed box but the gain of a ported. Would it work if i had the amp filter one above 25, and the other below 25 so they aren't overlapping? Has this been done before?
You'd do much better to choose a subwoofer that can be tuned in the low 20s, and do it that way. You can't hear anything below 20hz anyway, and the rolloff of a ported box will still allow it to play down low enough to cover any music material played that low.
"also if going from a sealed enclosure to a ported for one sub is a 3db gain, is porting 2 dubs a 6db gain (about twice as loud)?"
If you went from two subs in a sealed box and ported both, it would be an average 3db gain (generalizing a lot here, as actual SPL is dependant on too many factors).
As far as SQ, it depends on what you're looking for. It isn't as simple as sealed=SQ and ported=SPL. Sealed offers better transient response, but at the cost of higher distortion, lower power handling, and less efficiency. Ported has lower distortion, controls excursion more at tuning, and has more authority down to tuning, but at the expense of worse group delay and phase shift. In a car, sealed boxes offer flatter frequency response due to the cabin gain of the vehicle and the rolloff of the enclosure below resonance.