Here's the theory, as I understand it.
Yes, both speakers are commonly connected at the amp end, so theoretically the amp is seeing the same load as if single wired. That's the biggest claim by naysayers that bi-wiring is poppycock.
However, in practice, one wire sees the tweeter load, which has less of a power need, but could have a fairly low impedance dip somewhere. The other wire sees the mid/woofer which has a much higher power need but probably a relatively benign impedance load. Certainly each wire is feeding different frequencies.
In my experience, with my Apogees, it is fairly well known that bi-wiring is rather essential, and produces rather easy to hear audible improvements. Other speakers may not benefit so much.
I have heard that single wiring speakers in the following manner has some benefit. Connect the + to the tweeter + terminal and the - to the woofer - terminal (or you could try visa versa), then jump to the other unconnected + and - terminals from there. Others have reported sonic benefits going this way rather than just connecting to either the tweeter + and - and jumping to the woofer, or visa versa.
Experimentation. Yeah it's a pain but otherwise you'll never know.
