Bi-wirring long run rear surrounds

I recently purchased canare quad 4S11 speaker wire to biwire my (B&W 805's)surround speakers. The runs are long, 35 feet and 40 feet. I was told at a high end audio store that biwirring long lengths wasn't a good idea. It would be better to use two cables per speaker post or doubling up. Any one have opinions on this, I don't know which way to go.
Gaswho 52,
I have speakers with dual posts for bi-wiring. The manual mentions only about the benefit of bi-wiring. However, it never mentions about the optimal length of the cable for bi-wiring. I like to see what others have to write about this based on their experience. Regardless, the shorter the distance, the less degradation. In the meantime, I have not heard any thing wrong. My two rear full size speakers are bi-wired with approximately the same length as yours. I suspect that I have probably committed the cardinal sin already. :> )
Lej, just what is 'the cardinal sin'?

Gas, I THINK your store was addressing possible problems (eg less-well-damped bass) from the increase in cable resistance that results from single-biwiring, ie biwiring with parts of a single cable. Long lengths are already 'too long', and splitting the spectrum by single-biwiring increases each half's resistance.

I stumbled into some AudioQuest double-quad cable (part of their flat-rock series) that was really inexpensive and I'm double-biwiring my 32- and 36-foot lengths with it.
Gaswho52, I am running B&W Signature 7NT (in wall) speakers for my home theatre side channels.

After they were installed I experimented over several days, using two tube stereo amps in mono mode, both cables were run to the jumpers at the crossover of each B&W speaker (non bi-wire with jumper in place).

Next I removed the factory jumper and ran them as bi-wire, using four separate channels, making it bi-wire and bi-amp as well.

The results were much better with bi-wire plus bi-amp. Not certain this applies to your question as you may not have four channels of power available.

The only concern would be your run is too long for the conductor size (gauge). My runs are about 30 feet using Purist Audio and each conductor is 12 gauge (X4).

Let us know if you experiment and how it works for you. Would be of value to myself as well as other members.
Thanks for all the input, I'm going to try both ways and see what works best with my B&W 805's