ching, both CA751BD and Oppo do not support port multiplier. There is nothing you can do as it's not a computer and you cannot replace the eSATA card inside.
If you want to use 2 HDD together, use USB then.
However I have tried one movie before that does not run smooth on USB but runs smoothly on eSATA using the Century dock.
So it depends. Perhaps more c-hing can share their experience.