ping spawn & frugal experts!

[SemperFi]

Please forgive my lazyness, but reading the 1675 posts is just beyond the fifty words a day capacity of my mind...Not that my Q may relate to that thread...

How do you simulate a double-mouth enclosure like the Spawn in hornresp?

I assume I can sum the two mouths into one for the area, but what about the length? Either which way I've tried, the sims are aweful, nothing like the plots on the 'spawn' site. What am I doing wrong?

I'd like to build a bass cabinet using a single 15" Jensen (Neo15) for my daughter, and something like a double-mouth cab the size of an Ampeg 810 would be cool. Any comments?

Thanks