SATA is one device per channel, so you are right in that they don't compete like with 2 devices on the same IDE cable.

However, you are still reading from a single source. It sounds like the app could be doing a read for each burner, not one read for both of them. It is also adding more CPU and I/O overhead. If both burners were on the same IDE cable, it would probably take even longer.

Are both burners the same model? If not, they would burn at the max rate of the slowest drive.