But to answer your question - when you biwire - both of these wires still come together as one at your speaker terminals on your amp - so even if you biwire --- still 8 ohm load....This is true but if you measure the speaker terminals with an ohm meter you won't get a reading like Bluesbassplayer suggests. You may get an 8 ohm reading from the woofer but the tweeter will measure quite differently. There's more going on in the crossover than a simple resistance reading will tell.
Maybe someone else can explain or provide a source for a good explanation.