Biwiring your loudspeakers can really make a difference. Most people agree on that. But people seem not to agree upon whether the effect is good or bad.
Biwiring means using separate cable runs for the tweeters and woofers in which the cables run in a uninterrupted length from amp to speaker. It is not called biwiring if there is just one cable that is being split right before it connects to the speaker. Using a biwire cable that has 4 connections at both sides, you can also connect separate amplifiers to the woofer and tweeter, this is called bi-amping. The cable would then be called "shotgun".
If you have a biwire connection but are using single wire cable, just try running two sets of the same cable. You'll find that biwiring makes the soundstage larger, and gives more air to instruments within the stage. Some people find this unnatural and say that the stage should not be that high and therefore vote for single wire.
Please note: it's very important for the high-low coherence that you use the same cable on the low and on the high connections. Otherwise you might get disjointed, out of time bass and/or forward highs. There are people saying that the perceived larger soundstage is actually caused by phase problems introduced by differences in the cables for high and low. While this can certainly be true if you use different kinds of cables for low and high, it is absolutely not the case if you use identical cables of the same length or a biwire cable that consists of identical wires. I can confidently say that the benefits are always audible.
Now, whether you think biwiring sounds better is completely up for debate and very much a matter of taste. I for one do like it better. But then again, I like everything BIG.