Isobaric configurations

Vance Dickason's Loudspeaker Design Cookbook is a good basic reference for this stuff and will answer most simple questions. Well worth the price if you're going to be getting into building speakers. As Twl stated, they can be mounted back-to-back or front-to-front (these two are better than back-to-front as they cancel the motor system distortions better).

Yes, they can, but most of the ones I've seen use sealed cabinets.
There are several Isobarik configurations that I've seen.
They include:
Face to face - out of phase,
Back to back - out of phase,
Front to back - in phase.

I have also seen a TL tube configuration which uses a front to back driver orientation, either in phase or out of phase, which has a long TL tube extending from the Isobarik chamber between the drivers.

They will use more power because you have to drive 2 woofers.

As far as using 4 ohm or 8 ohm voice coils, that depends upon your amp's capabilities to drive low impedance loads.

Perhaps someone else can shed more light on this subject.