I think you will be able to connect to the HDD via two plugs, yet I don't think it will make any difference. And no, neither will you get faster speed. I would recommend connecting via FireWire400, if the HDD supports it, go for FW800.😉
Though USB 2.0 goes up to 480mbps, and FireWire400 goes up to 400mbps, FireWire is more stable, and USB dosn't always run at 480mbps. And, you'll save a USB port.😉 FW FTW! (FireWire For The Win!😀 )