Look at the back of each drive; there should be a jumper that for master, slave or cable select.
Either set the first drive for master, the second for slave or set both to cable select.
If the drives don't have the jumper settings labeled, either break out the manual or check the manufactures web site (some companies put the label in a nice, easy to see spot right on/by the jumpers, some put it on the drives surface (top/bottom). Some are easy to tell, like WD's drives, some, like Maxtor, make it a pain in the a$$ if you have multiple drives installed).
On a side note:
What burner software are you using? Keep in mind that most software will only work with the drive it was bundled with. Your old software may (probably) won't work with the NEC. If your NEC didn't come with any software, you can either buy new software, upgrade what you've got, or use one of many freely available burning apps.