I have 2 HDDs, one primary master, the other primary slave, and 2 DVDRWs, one secondary master, the other secondary slave. Originally it was a CDRW and a DVDRW. No problems at all, even writing 'on the fly', albeit it is safer to write via HDD.
So you just have to decide which is going to be a master and which a slave and set the jumpers accordingly.
If you look at the Installation guide when you purchase your New DVDRW Drive (dont be mean buy the lad one too (under £50)) youll find jumper settings a doddle.without droppin a stitch. I use DVDRW&CDRW as m/s on same IDE cable & 2 HD on other cable.. you could also add a PCI IDE card to put the DVDRW drive on, although thats a far dearer option..