So how did he design them? What tools does one use to do this? I have a form factor in mind for some speakers that I'd like to build for my off-road truck [1]. I have a certain look and mounting style in mind and I haven't seen anything like it. I just don't know where to start with the design, though.
In addition to what jzwinck recommended, I'd also recommend the Loudspeaker Design Cookbook [0] and the Master Handbook of Acoustics [1]
If you're an engineer that's comfortable with differential calculus, I'd recommend the grandaddy of them all: Acoustics by Beranek [2] (I'm actually linking to Acoustics: Sound Fields and Transducers which is an updated version of Beranek's original Acoustics cowritten with Tim Mellow)
I made some speakers when I was in high school. The tools I used were a table saw, a whole bunch of long clamps, a screwdriver, and a hand drill. But most important, this book: http://www.amazon.com/Building-Speaker-Systems-2nd-Edition/d... . If you buy it used now you will find it hilarious how they describe using computer software to analyze speaker systems, but no matter: you can stand on the shoulders of giants and design your own boxes without any software at all.
The "D'Appolito array" is covered in that book--a design employed in many store-bought speakers where two mid-woofers flank a tweeter at a specific distance to avoid some of the time alignment concerns raised here. This is a good candidate if you want to build smallish, narrow speakers for home. In the car it usually comes down to "where is there enough room for the drivers I want?"
I think the book also discusses various dual-driver arrangements for subwoofers, which is another interesting area where you can use twin drivers to eliminate undesirable effects. If you put that together with the D'Appolito design in a single cabinet, you will have something like what Infinity used to sell for $1000 each...but doing it well won't save you much money because good drivers are expensive.
[1] https://flic.kr/p/o1xmJ8