connecting speakers in SERIES will increase the impedance, running them in parallel will decrease the impedance. Since you are using 2 identical speakers, connecting them in parallel will result in the impedance being cut in half.
WIth the x-over on a bi-wireable speaker, I don't know if you can connect in series...
One possibility, and this would take a lot of pieces of speaker cable, would be to connect the 2 woofers in series, the two tweeters in series, and connect those two legs to the amplifier in parallel. This would require 3 pairs of cable, probably cheaper to sell your existing speakers and buy a center channel than buy 2 more pairs of cable. Also, I haven't the slightest idea how it would sound, but it should create a more stable impedance for your amplifier.
If you want a quick sketch of how to connect as described above, send me an email with a fax number, and I will present my non-artistic rendition of the schematic.