[ElDiabloConCaca]

You get 480Mbit per USB "chipset," and on most Mac computers, there are one or more USB chips controlling one or more USB ports. So, you could have two USB ports operating off of one USB controller chip, so there would be 480Mbit of bandwidth shared between those two ports. Telling which ports operating off of which chips, though, is beyond me.

It really doesn't matter, though, since 480Mbit/sec roughly equals 60MB/sec, which is something a standard, one-drive, USB enclosure just won't do because of the real throughput of the hard drive in the case (more than likely, you'll never see more than about 20-30MB/sec transfer rates per hard drive). So even if two hard drives were connected to "shared" USB ports, you'd still have trouble saturating the 480Mbit throughput of the USB chip. I doubt you'd see any real slowdown using two hard drives on one USB controller chip than you would using two hard drives on different USB controller chips.

Now, throw a third USB hard drive into the mix, and we may have some theoretical USB throughput hubbly-bubbly to argue about, but with two hard drives, I think you'll see perfectly acceptable transfer rates.