Copying on the fly is almost impossible for DVD, as most drives read in CAV while most write in P-CAV or CLV. Hence, the reading will be slower than the writing for at least the 1st GB or 2. So you must lower burn speed, usually to 4x or sometimes to 8x (if the reading drive has no riplock). Thus, it's usually as fast, or faster, to rip to HD then burn at full speed.
For dual layer video discs, many drives read at 8x CAV, making then too slow even for 4x CLV burning.
Obviously, you cannot copy on the fly with one drive. But to answer your question, there's not much point in having 2 drives in this instance. Unless you have them on separate IDE channels, and want to use one for ripping while the other is writing.