Single IDE vs Dual IDE?

jrsimmons asks: "I'm running performance tests on IDE interface configurations for my company. I've discovered that disk to disk I/O is significantly faster (in the realm of 30%-40%) when only a single IDE interface is active versus when two IDE interfaces are active. This is significant as our servers are used to provide Point-of-Sale availability for registers in the retail environment, which is heavily dependent on disk i/o performance for efficiency. I have run the tests under both Windows and our retail OS (sorry, no Linux) with similar results. What are some possible explanations for the detrimental effect the second active ide controller has on disk I/O speed?" Has anyone measured this deficiency on Linux and other Unices?