Passive Crossover Network

Connecting a 2-ohm speaker to a crossover network designed for a 4-ohm speaker will double the crossover frequency point for the lower impedence driver.  For example, if the x-over is designed so the woofer to midrange transition is at 800 Hz, connecting a 2-ohm driver into the woofer section will double that X point to 1600 Hz.  If there is a 4-ohm speaker used in the mid section, it will remain crossed at the lower frequency.  So, between 800 and 1600 Hz you would have two speakers connected in parallel, a 2-ohm and a 4-ohm, for a net load on your amp in that frequency band of 1.33 ohms.  This will do two potentially nasty things.  First, it may overload your amp if it is not stable at a 1.33 ohm load, and second you will have a terrible response peak in the band from 800 to 1600, which will make your system sound like a telephone.  (well, not that bad, but it will enhance the telephone-like sound of vocals and other midrange sounds.)

In other words, I wouldn't do it.  :)