Only under very extreme conditions, Forrest. If those two lines aren't even hooked up to the same router or computer, then there shouldn't be any chance of interference. My guess is that you're probably measuring your download speed off of a p2p program like LimeWire, Bear Share, or Shareaza. Don't. Those are misleading because they depend on other people's upload speeds.
If you are actually taking this from an official download off the internet that was done over a web server, then it might be that the server has a capped bandwidth. Try going to one of those internet speed measuring sites