>>Maybe a good place for me to start is with a book or internet resource, to better understand using flash (and it's limitations, etc).<<
Yep. But since you've got me engaged in this thread, I'll try a short answer... ;-)
First, I'm not a flash guru and I rather do without, since the flat lighting coming from the camera usually destroys what made me want to take the picture in the first place. But then there are situations where either I need to just fill in a little to lighten up shadows or where I want to freeze and capture a moment which would otherwise have been lost. The latter is almost exclusively restricted to people photography.
As for architectural photographs I strongly recommend the use of a tripod, instead. Period. Step down as much as you like or need, maintain the original atmosphere of lighting (which is a vital part of the architectural design) and forget about complicated flash set-ups which, BTW, the Xpan (being non-TTL, flashwise) isn't designed for.
Just a last short note: Don't try to widen the angle of coverage by having two units overlap. You won't be able to control the amount of intersection precisely enough, which will inevitably leed to overexpusre in said area.
Addendum: There are light sensors available to trigger secondary flash units in "slave" mode. You would have to arrange and distribute these secondary flashes around and/or about the place in order to pop off automatically as soon as your main unit fires.
Did I mention that for architecture you should get a tripod, instead?