Also note that impedances are averages; i.e. given different loads a speaker will change resistances.

This leads me to two comments:
1. If you do this, make sure the speakers are twins, not just nominally 8 ohms.
2. If you can, get a resistance chart for the speaker model (like the M80ti one Alan so helpfully linked to in a recent comment). If the chart shows a big dip in resistance somewhere, don't wire them this way. The severity of the dip will be twice what it normally is if you do.

Hope this makes sense- basically, I'd recommend this only on identical speakers and only if the speakers are a solid 8 ohms given any load.