Right, i'm not 100% on this, but i've played RE2 quite a few times, so i'm pretty sure it's somewhat accurate.
If you don't use the cord at all, then zombies will come out of both sets of windows, that being the licker corridoor and the one leading to the basement.
If you use it in either of the corridoors, the exact same corridoor is the one that seems to get overrun (which is a bit odd to me, it's like they ignore the fact the windows on the other side of the building are like, non existant)
So for example, if you used it in the licker corridoor, that one would be overrun, whilst the other one wouldn't.
You also get the cord VERY soon before the scene, so you don't have long to use it, and it always, always activates in the 2F library, so you have to activate it, there's no avoiding it.
You can't use it in the B game, it doesn't appear, but it does affect the B game in the same way the A game was affected, so ignoring the cord is not recommended unless you know what you're doing, as most people find the B game tricky enough as it is.
I think that's right anyway, hope that helped. ^^
Edit: Oh and yes, there's only one cord. =P